If the cast members are doing their jobs, this won't work. When Group 1 enters the FastPass line and requests rider swap, the cast member is supposed to have Group 2 "burn" their FastPasses as well. Otherwise you can double all of your FPs by having Dad book rides A, B, and C while Mom books D, E, and F, then they switch off and essentially get six FPs each.
In my experience, this is almost never enforced at WDW but it was enforced regularly when I visited Disneyland in February.