Presumably it's best not to apply the two too close together. minoxidil needs time to lie, undisturbed, if it's to be absorbed, and I wouldn't fancy spreading minoxidil onto scalp still coated with an obvious layer of retin-A cream.
Perhaps use the retin half an hour before minoxidil application, or an hour, two, three... whatever feels alright and suits your timetable - after the minoxidil dries would be fine too. Oh, and only one application of the retin each day.
Slarti