I would not even consider putting another bird not even another grey into the cage with Belle, greys as a rule do not get along with other birds not even their own kind unless they are clutch mates.
Greys need their own cage to call their own, their space that they feel safe in and even if the new bird and Belle would get along they still need separate cages for you never know when the time will come that one of them decides to take a bite out of the other, spats can and do occur.
I know you want to cut out having to get yet another cage and heaven forbid another one to have to clean but this is the only way.
I don't think you ever introduced yourself in the welcome room, why don't you do that and tell us some more about you and Belle, maybe some pictures too.