How do you carry two stacks of the same unit?

[KingImp]

This is really starting to irk me that either I can't figure this out or this game won't allow me to do this.

I have a whole bunch of Priests and Inquisitors in my army. I figure why bother even keeping the Priests if the Inquisitors are their upgraded version, so I want to place the priests in a Garrison and then split the Inquisitors I have into two stacks. Well, it won't allow me to split them. It keeps placing them back into the same stack.

Enemy armies have no problem carrying two or more stacks of the same unit, so why can't I?