When you are setting them up, you can get the same stem, but you may or may not need the same spacers due to head tube height. If one is way out from the other, then you may need a different size frame and a longer/shorter stem.
It's tricky, but so long as the frame measurements are within near enough then you will be fine.
I've got two 56s in conventional old Skool, but 54 in modern geometry. The two 56s are a little different, but ride very similar with positions.