I'm sure someone will tell me to do a search, but I can't figure out on what

For a mid-May graduation weekend in southern California, I've gotten a very good rate at a Doubletree.

I know that Mrs. cblaisd and I will be needing a room, but another family *might* be needing a room (but probably won't know until early April).

Should I go ahead and book two rooms on one reservation? Or book two separate reservations? I'm concerned that if I do it on one, and come mid-April they don't need the room, then re-doing the reservation and dropping one room is likely to make the re-done reservation subject to a much higher price.

Otoh, doing it on one reservation means (I assume, but don't know) that both rooms will get the benefit of Diamond upgrade? Or maybe not. I've not encountered this one before.

Advice?

Thanks!