I have this problem quite often- I'm 6'2", so if I do a group cosplay, chances are that the heights will be off. Its hard to completely reverse which cosplayer is taller without having one sit or stand on a raised platform, but you can make the difference less obvious by having the taller person slump or slouch. This only works if poses like that make sense for the character, however. Additionally, if posing near a wall, have the taller person lean back against the wall and slide down a bit, while the shorter person stands straight up.
It isn't that big a deal, though, so don't worry about it too much. Do make sure you practice posing together, though, and if you come up with any other clever height-minimizing poses, please share!
Erza Scarlet, flame hakama outfit, Fairy Tail 90%
Neliel Tu Odelschwank, Bleach 5%
Shiina, Angel Beats (postponed)
Mulan, Mulan 5%