Interesting idea forming here. :) Keep in mind though that not all twins are alike. xD I don't really see the purpose of twins if they are going to be seperated other than them being born on the same date and just being twins. xD
Some things to think about...
1) How will you select twins? Like, both at the same time or separatley? If separatley, then one can be dyed/beaned while the other is left natural/whatever colour.
2) How will twins sell? Will they be of extra worth? You realize that twins don't have to be on the same account, don't you? x3 They can be separated and have that thing that says "This pet is twins with _______" or whatevah. :P
3) Will they always have to be the same gender? Cuz not all twins are. :/ They also don't have to be the same colour.
It would be really cute and I would 100% support if the twins were, like, 2 creatu in one pic. But since it's gonna be either 2 big or small and cramped up, I don't support AS much.