I have a bonded pair made up of two entirely different species. It happens sometimes but is NEVER something you can count on and as
@hrafn said you absolutely cannot cage them together under any circumstances. No matter how close my pionus and poicephalus are, they still have frequent misunderstandings and could seriously injure each other.
Honestly, you can't even bank on two birds of the same species getting along and I'm personally against housing more than one bird in a cage
at all since no matter the size it's still an enclosed space that lends itself to escalating potential conflict. But if you must keep two birds in one cage, then it is imperative they're both the same species
and good friends if not mates.
That said, it is entirely possible to have birds that are bonded to each other and still seek out your attention and see you as more than just a food source. I have a good relationship with my bonded cockatiel pair and both birds in the aforementioned inter-species pair. I would recommend getting one at first and establishing a bond with them before bringing home the second. You can build a bond separately with the second bird during the 30-day quarantine process, longer if needed, and then introduce them to each other.
I don't have any experience with green cheeks, but most of the cockatiels I've dealt with have not been picky when it comes to making friends with other cockatiels... with one exception who wanted to destroy anything that moved.
A bird who gets most of their companionship from another bird is going to be happier and more mentally stable, and with a bit of effort you can still be an important part of the equation.