As long as he’s listed on the DVC reservation, you’ve done nothing wrong! I didn’t know if you could be on two at once, obviously you can. I don’t think you can be the lead name on both.
As for bands.... these just access his MDE account. That’s where his tickets and FPs are stored. So both bands will have his tickets and fastpasses, as long as his name is identical on both reservations. If not, you may have a problem.
I don’t know how his resort reservations will be “managed” by the bands though. I’m not sure if one band can unlock two doors, it might be that he will need to use both, depending on where he is. But a CM should be able to see he has two reservations.