Should be no problem. A friend of mine went to Universal/IOA every trip to Disney. One time she got a ride back to Disney from a friend. The next trip, she used the return ticket to get to Universal, then purchased another roundtrip for her ride back to Disney, so was using the tickets in reverse. So I don't think it matters when you use the tickets.
However, if you do this, the overnight bag you carry needs to be small enough to sit in the seat with you or at your feet, as those shuttles will have no luggage assistance.