It might be a hard adjustment for them to make, but it could probably be done if it has to be. If you can really only afford the one, then finding a good home for the other is what is best. I know money can be a huge issue. But there are low cost vets and other options that members on this site could help you with, if that's your only obstacle. However, you mention not wanting 2, so I don't know if there are other factors for you.
It's really going to depend on their temperments and how bonded they are. Some cats might shrug it off while others would be permanantly miserable and could develop real behavioural and physical problems. How close are the cats? Maybe you could find another home for both of them together?
I would say, if there is any way you can keep them together, than do it (even if it means trying to find them a home to go to together). If it's just not possible, then split them, but take the time to find a good home and interview the new family well, so you can be confident that he/she is well taken care of. Keep in touch and be prepared to reunite them if one or both take a really bad turn after the split. It would be hard to find a home for both of them together, but it might be the best thing if you're completely unable or unwilling to care for both.
You're a great guy for caring enough about both of these little guys to ask this question and do what you can for them! Keep us updated on what you decide to do!