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If you're hams are getting along fine together then you don't need to separate them Stocks although it is a good plan to have a second cage ready just in case they do fall out at some point as this often happens as they mature & want their own space, if they do need to be separated they live happily alone so don't worry about that.
While they're together the general advice for pair cages is a bit different to single hams, you don't want anything too big & it needs to be a single level with no tubes or areas they may get territorial over, there are tank style cages or barred cages that would be suitable.
For tank style cages something like the Duna multy would probably be good or for barred cages perhaps something like the Hamster Heaven (without using the tubes or penthouse) or the Savic Micky, these are all a good size without being too big & would be suitable for a pair set up.