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Sorry for the long post.

I have four birds, two cockatiels, and two budgies. I also plan on getting a Senegal sometime soon.
First, is it hard to take care of more than one parrot?
I don't think it's any harder to take care more than one parrot. Actually i think it's a little easier. They keep each other company. Even though the budgies and cockatiels are in separate cages, they are still in the same room so they can chatter to each other. I go to school during the day, so i don't have to worrey too much about them getting bored. So i think it is some what easier.
Though like Tracy said, it is more food, more noise, more vet visits, etc. That's just part of owning a pet, any pet.
I would research a number of different species your interested in. Sun Conures are gorgeous, yet LOUD, they were my first choice. I don't know your living situation, but if you live in an apartment, or condo, or town house, or anywhere that restricts noise, i would go with something else. Tex and Christy do live in an apartment, though i think they were really fortunate with both the bird and their neighbors. I personally wouldn't take the chance if i lived in an apartment which is why i didn't pick a sun conure since when i move out i'll most likely be renting somewhere, and most rentals restrict noise.
If you live in a housing situation that restricts noise, don't worrey there are still a lot of gorgeous birds out there that are not too noisy. Green Cheek Conures, are fairly quiet, as well as a senegals, black capped conures, parrotlets, pionus, the list goes on. I think any of those birds i said above you'd have a lot of fun with.
Can you give enough attention to 2 parrots?
Well i don't think i can answer that, as it depends what situation you are in and how much free time you have.
Yes it is very possible, even if you work or go to school during the day. But if you do other activities, or if you work late then maybe you cant.
If you work at home, or don't work at all or something like that, then yes, i think you could manage 10 parrots. You just have to ask your self, how much time do i give my cockatiel, how much time do i have left over, how much time am i willing to give etc. If you get off school or work at 3 or 4 and come home and take them out then yes i think you can manage 2.
Do you think my cockatiel and the sun conure will get along? (My cockatiel is a female).
I can't say, it's a risk when getting another parrot. Though i have to say, even if they don't get along you can still manage. Just make sure you get it as a baby, hand fed and they each have their own cage and personal space and i don't think it should be problem. Specially if you introduce them well and don't force them to be together.
And what else do I have to know about taking care of two parrots?
Make sure you quarantine your new parrot. Meaning put it in another room if you can for about a month. This is just in case it is sick, you don't want your tiel getting sick either. If you live somewhere where you don't have space to quarantine you can take it to a avian vet as soon as you get it and that should be find.
Make sure when you have them out together you watch the closely. If they show any aggression towards each other then you should separate them asap, to avoid one getting hurt, or worse.
And i think thats all.

I'm sure i've missed something that others can add on.