If you got both at the same time from the pet store then it wouldn't matter if you had a boy or a girl, they'd be used to each other.
If you get one first, then it depends on the ferret's nature if it will accept a newcomer. Some ferrets are happy with having a new addition while others aren't happy at all.
But I really don't think it matters if you have two boys, two girls or one one each. They're all great and will be excellent pets to have.
