Student or Learner
Suppose, my friend and I were sitting on the chair in the house, tooking off our shirts. Suddenly my friend stood from the chair and wore his shirt to move out. In such a situation, can I ask him, "Where are you off to"?
If you look at your first sentence, you are both sitting on one chair [the chair in the house]. You do not need a comma after "Suppose".
"taking off" - which means you are carrying out the action of removing them.
He may well be wearing his shirt, but he has to put it on again.
"Suppose my friend and I are sitting on chairs in the house, without our shirts. Suddenly my friend stands up and puts on his shirt, starting to walk out. ...."