You are free to use anyone's FP+ reservations - we switch ours around all the time. DH and I aren't big thrill riders, so we let our teens use our magic bands and ride the headliners twice.
As of April, you can use other people's magic bands, even if they haven't been used for admission. This is a loophole that Disney is aware of, and according to another poster, is going to close sometimes this year. This worked for us in November, December and April.
Sometimes, our family will split up and just our teens will go to the parks alone. DH and I just give the boys our magic bands and they use all of our FP+ reservations - even though we are still at the resort.