Thia is a stage most children go through and it is tough to stop when you have 2 in the same stage at the same time. You MUST discipline them!
I know that they may seem too young for discipline, but they are not. I have 3 children who have already gone through that stage and one more still to do so.
Is it possible to put them in separate rooms at nap time? If so, I believe it will be easier to keep them from doing this as it appears they enjoy playing with each other and copying each other.
If the discipline you use and separating them doesn't work after a couple weeks (You must be consistent during those weeks!), I recommend using duck tape and taping over the snaps on their onsies when you put them on the children.
HTH & good luck!
Have a GREAT day!
S.