As I know, there are serveral ways:
Firstly case, for a short haul flight (e.g. 2 hours) return on the same day, they may just be the same pair of pilots..
Second case, if it's a long haul, the B767 pilots may fly the MD-11 back to the origin. They may seat at cabin.
Or sometimes, they may fly by other airlines to a destination that have B767 flights go back and fly on that route.
There maybe more cases..
Hope this help.