I'm not sure where you're getting the impression that people can't ride twice. A few people have posted that that is allowed.
For instance, a family of 5 with Mom, dad, 2 tall enough kids, and 1 kid who is too short. Mom and the 2 kids tall enough are going to ride first while Dad stays back with the short kid. Dad and 2 tall kids get a rider swap pass added to their bands. Mom and 2 tall kids ride while Dad takes the short kid to get a snack. Then mom stays with the short kid while Dad takes the older 2 back on the ride with the rider swap passes. Mom and dad each ride once and the two older kids get to ride twice, once with each parent.
In regards to everyone needing a FP, there have been several reports of the second group not needing it. Some people have posted that phone/chat/emails with Disney have indicated that everyone needs a FP, but others have said they were told the second group didn't need FP. So far, I don't think anyone has reported being denied a rider swap because the second group didn't have FP.