Okay, so they are old enough and feathered enough to not need to have any additional external heat source, and they should be starting to wean onto solid-food at this point like Christa said...
To answer your question, YES, you absolutely MUST feed them their formula between the temperatures of 104 degrees F at the coolest, and 110 degrees F at the hottest...If you feed them formula that is not at least 104 degrees F it will cause Yeast to grow in their Crops and throughout their GI Tracts, and this will cause "Crop Stasis", which means their Crops will stop emptying, and this will eventually kill them. This is the #1 reason by-far that baby parrots die while being hand-fed by someone with little to no experience, because they don't use a digital cooking or candy thermometer that has a metal-probe that you put in the formula and keep it there while you're feeding them, and re-heat the formula as needed if the temperature falls below 104 degrees F...And any hotter than 110 degrees F can cause burns to their Crops...
***Most people who have never hand-fed a baby bird before simply mix-up the formula with warm/hot water coming out of the tap, and the average temperature of that is in the 70-80 degree F range, so WAY TOO COLD...This is why the often become sick and die before weaning...So you really need to measure the temperature of the formula every time you feed them...Also, they usually don't eat their formula with much excitement or feeding-response if it's not at least over 100 degrees F anyway...
***WHAT ARE YOU CURRENTLY FEEDING THEM AS FORMULA??? This is extremely important as well, because if it's not a commercial hand-feeding formula for baby birds, but rather something like homemade formula made from crushed-up grains, seeds, etc., they haven't been getting the nutrition they need and they can have severe health and metabolic issues, as well as issues weaning onto the solid food...And if you feed them some type of human baby formula or cereal you can kill them quickly because they are loaded with added Iron, because human babies need lots and lots of Iron; however, birds cannot handle much Iron at all in their diets, and if they eat too much of it they develop something called "Iron Storage Syndrome", which is fatal. Human baby cereals such as Cerelac, Simulac, etc. are often used as hand-feeding formula for baby birds in certain countries, and often the babies die and the owners either don't understand why, or they just figure that the baby had something wrong with it like a disease or infection, when in-reality they are dying due to the incredibly high Iron content in these human baby cereals and formulas...
As far as how hand-feeding them, based on their ages I'd say they should be getting between 3-4 hand-feedings a day with the hand-feeding formula, but they also need to be living in their first Weaning/Starter Cage, and whatever staple food you are planning on weaning them onto, either an Avian Pellet or an Avian seed-mix needs to always be in their Weaning-Cage at all times. I don't know if they've already started to eat any solid foods yet or not, but at this point they always need to have their pellets or their seed-mix inside of their Weaning-Cage at all times...You should also be offering them fresh Veggies and dark, leafy Greens every single day at this point as well, because if you don't introduce them now they won't eat them in the future. Same thing with fresh Fruit, you should be offering them a bowl of chopped-up fresh Veggies, dark leafy Greens, and a tiny bit of fresh Fruit (lots of sugar, so not too much) every single day, once a day, while they have a bowl of either pellets or seed-mix all day long, every day...
****It's extremely important that you "Abundance-Wean" them, which means that you allow them to fully decide when the amount of formula they are eating in each hand-feeding is reduced, and you allow them to fully decide when an entire hand-feeding each day is eliminated...You never want to make those decisions for them, as this is called 'Force-Weaning" them, and it will result in severe neurological and behavioral issues for the rest of their lives. So I'd be offering them between 3-4 hand-feedings of formula every single day, once first thing in the morning when their crops should be completely empty. After they have their first hand-feeding of the day first thing in the morning, then you should put a bowl of pellets or seed-mix inside of their Weaning-Cage, along with a bowl of water.
The second hand-feeding of formula should be around noon, but you need to always check both of their Crops BEFORE each time you offer them a hand-feeding, and their Crops should be almost empty before you offer them their next hand-feeding of formula. They will eat the most formula during their first hand-feeding of the day first thing in the morning, and then once you put the pellets or seeds and water into their Weaning-Cage after their first hand-feeding of the day, they will start to pick at the pellets/seed-mix. So keep checking their Crops, and they should be almost empty right around noontime, or about 4 hours after their first hand-feeding of the morning (their Crops don't have to be completely empty in-between their daytime hand-feedings, but almost empty. The only time their Crops will be completely empty before you offer them a hand-feeding is going to be first thing in the morning, because they've gone overnight without eating)...
After they have their second hand-feeding of formula around noon or 1:00 p.m., which they may or may not eat a bit less formula than they ate during their first morning feeding, then make sure they still have pellets or seed-mix in their Weaning-Cage...The third hand-feeding of formula you're going to offer them should be around 4-5 in the late afternoon, making sure their Crops are almost empty. They will probably eat a bit less in this feeding than they did in the other two previous hand-feedings...After they have their third hand-feeding of formula, that's when I would put in a bowl of fresh Veggies and dark, leafy Greens for them to have in addition to their pellets or seed-mix....
Their last hand-feeding of the day is going to be around 8-9 at night, right before they go to sleep...This hand-feeding will be the very last of the day, and they will eat the least amount of formula during this hand-feeding. Then they go to bed...
***The amount of formula you're going to feed them during each feeding is going to be determined entirely by them at this point, as they are old enough and large enough that they are most-likely not going to gorge themselves and over-eat on formula. So they will more than likely let you know when they're done with a hand-feeding, as their feeding-responses will stop when they're full, and they will reject the syringe and stop opening their beaks up for it...You still need to always check the size of their Crops while you're doing a hand-feeding, so that you're sure that you're not over-feeding them...Again, the size of their Crops after a hand-feeding should be like a balloon that has some give to it when you gently use your finger to feel how full it is. Their Crops should never be tight or have no give to them at all...At their age they should stop eating at an appropriate time, but you still need to always check, and if their Crops are very huge and tight to the touch with no give to them, then you need to stop them from eating any more formula, but that shouldn't happen often at their ages...
They are quickly going to choose to eliminate a hand-feeding, but you need to not only allow them to make that decision, but you need to keep in-mind that they may not both Wean at the same pace or the same time, and even though one of them rejects a hand-feeding, the other may still need it. The first hand-feeding that they will eliminate will probably be the one at either noon-1:00, or the one at 4-5 in the afternoon. You'll know they are eliminating a hand-feeding because not only will they reject the syringe when you give it to them by turning their heads away, not opening their beak, not having any feeding response, etc., but also because they will noticeably eating more and more of their pellets or seed-mix, and their Crops will be too full to eat a hand-feeding anyway. But again, you have to let THEM decide to eliminate a hand-feeding...
Usually what happens is they will eliminate the 2 hand-feedings during the daytime, the ones at noon-1:00 and at 4-5. They'll continue to want their first hand-feeding of each day first thing in the morning for quite a while, and then they'll eat more and more pellets/seed-mix and Veggies/Greens during the day, and then they will typically want their last hand-feeding right before bedtime around 8-9 at night, but they won't eat much during this last hand-feeding of the day...This is more or less just them psychologically wanting to be fed before they go to sleep, and it will be the very last hand-feeding that they will eliminate...
So eventually they will stop wanting their first hand-feeding of the day and will just start eating their pellets/seed-mix first thing in the morning. At that point the only hand-feeding of formula they will want will be the small one right before bed around 8-9 at night, and they typically will actually cry and whine for it...This is called a "Comfort Feeding", and it continues after they are basically fully-weaned and eating only solid food. They just want the "Comfort Feeding" before bed as a psychological thing, and they won't eat much. At the point when they stop whining at night before bed for a Comfort-Feeding, you should still offer it to them, and at the point where they stop whining and crying for it and you still offer it to them and they totally reject it, and the do this for a good few days to a week without eating any formula at all, even right before bed, that's when they are fully-weaned...