Birds are flock animals and should always be kept in pairs or groups.
IMO, "bonding" is overrated. 1 of my birds is bonded to me so when I go to work, he screams and gets upset. My birds that have no bond to me don't care that much. They still hang out with me and like me, they just aren't obsessed with me, and I see it's better that way. But either way getting 2 birds won't keep the birds from bonding to you. One of my tiels has been trying to bond to me lately and she's already bonded to a bird.
If they are already living together I'd keep them together. It depends on the birds, my tiels are in stacked cages, my lovebirds live together, and my conures are in cages next to each other. Because my tiels don't do well living together but as soon as the doors open they can be together, the lovebirds have no problem being together 24/7, and the conures only want to spend a couple hours a day together.
While I respect this opinion, I completely and totally disagree with it, and here's why:
Yes, birds are "flock" animals, this is true...HOWEVER, the idea is supposed to be that YOU ARE THEIR FLOCK!!! As Itzjbean has already wisely stated, if you start out by buying two Cockatiels, or any birds at the same time, chances are that they are going to bond closely with each other, and may or may not have any bond at all with you. This especially applies if you are thinking about bringing home a very young or baby bird, as two babies have a much higher probability of bonding closely with each other and then totally pushing-out any human companionship (where as with adult birds you really can't guess how they are going to interact with each other, they may hate each other, they may just tolerate each other, they may bond closely with each other, you just don't know, but with very young or baby birds they seem to more-readily bond closely with each other).
***You absolutely DO NOT want to bring home two young/baby birds at the same time IF your goal is for YOU to bond very closely with your bird, and for YOU to be your bird's companion and "flock". And again, as Itzjbean wisely stated, after you bring home your first bird and you bond-closely with it and you form a close relationship with it, then in the future if you want to add a second bird BECAUSE YOU WANT A SECOND BIRD TO BOND WITH AND TO JOIN YOUR FLOCK, then you should think about bringing home a second young, baby bird or adopting an adult bird (NOT because you want to bring home a second bird for your first bird, because again, you will have no idea how two adult birds are going to interact with each other, so YOU need to want a second bird companion if you decide to do this)...
Let me emphasize something to you though that is very important (and from what you describe it does really sound like you know how much of a commitment you are making)...If your main goal is to bring home a Cockatiel and for them to bond closely to you, to be your companion/family member/child, however you want to think of it, then YOU must make the commitment to spending as much time every single day as you can with your bird, just as if they were your actual child. Parrots are extremely intelligent, having the intelligence of a 3-4 year-old human toddler, and as such, they can easily become bored and understimulated mentally if you get tired of them or you fail to include them in your life. So it's extremely important that you locate your bird's cage in the "main" room of your home, the room where the "action" is, so that your bird feels like they are included in your life and are a part of your life. Usually this room is the living room, family room, TV room, etc., whatever room of your home is the room that you spend the most time in when you are home, and the room of your home where other people usually are when they visit. Even if you're not directly interacting with your bird and they are inside of their cage, just being in the same room as you while you are reading, watching TV, playing video games, talking, etc. will not only make your bird feel safe, secure, and a part of your family and life, but they are more likely to happily entertain themselves inside of their cages with their toys...What you don't want to do is locate their main cage in a "spare" room or "back" bedroom that is like a dedicated "bird room"; a lot of people think this is a nice thing to do, but one of the things that frustrates a bird more than anything is being able to hear you and know that you're home, but not being able to see you and not being in the same room as you. This is what usually causes constant screaming, as well as other issues.
If you are both WILLING and more importantly you WANT to become your bird's close-companion, your bird's "person", and you stick to your plan that you wrote in your post (which is a good one) of spending time with him before work in the morning, then after work in the evenings, then on all days off, eating meals with them, taking them places with you, playing with them, and letting them out of their cage whenever you are home, and you can provide them a nice, large cage with tons and tons of toys and foraging activities to keep them happy and healthy and entertain them while you're at work, and you really want to be your bird's "flock", then yes, by all means, bring home a Cockatiel, as forming a strong, close bond with a parrot is like no other relationship you've ever had before, it's extremely rewarding for both you and your bird.
And again, if you want to form a very strong, close bond your bird, then DO NOT bring home two birds at the same time to start out with. Yes they are "flock" animals, but the idea is that YOU are going to be their flock, and after you form a close bond with your bird and you guys become your own flock, then you can certainly add to your flock later on, as it is very true that if you bring home two birds at the same time they are very likely to bond closely with each other and not with you. That's just nature. And this is why it's extremely important that YOU bond closely with your first single bird, and that you are their companion.
Just to give you a pretty good example of doing this and how it can work out very well and accomplish what you're trying to accomplish, I currently have 4 parrots that I got all at different times, and who are all extremely closely bonded with me, I'm their "person" and their closest companion in life, however, they all are in our family flock together. I first got my Cockatiel, a hand-raised, baby female, and I had her for almost a year by herself, bonded extremely closely with her, and then about a year later I brought home my Quaker Parrot, a hand-raised baby female. I again bonded extremely closely with her, and because my Cockatiel was already over a year old and bonded closely with me, she wasn't really interested in bonding closely with the baby Quaker, and the baby Quaker was enamored with me. Then about a year after bringing home my Quaker I brought home my Green Cheek Conure, a hand-raised, baby male. Again, my baby Green Cheek bonded very, very, very closely with me, he was attached to me constantly and still is, and as time went on he became pretty close to my Quaker, and her to him. So now they are very bonded to each other, but they are both closely bonded to me first, nothing has changed that...And finally about 7 months after I brought home my Green Cheek I brought home my Senegal Parrot, a hand-raised, baby male. And this guy attached himself to me and never let go. He's my sweetheart, and while he doesn't hate the other 3 birds, nor does he ever become aggressive or violent with them and he knows that they are a part of his flock and he accepts them, he could really care less about any of them, and he just wants to be with me. And now we're all at the point where we can all be together as a family, or a "flock", hanging out, watching movies, eating our meals together, etc., and they're truly like my children. They're MY FLOCK, and I'm theirs, and we love it this way.
and he has a couple Amazons AND an African Grey 
