Thanks for the prompt reply travelinfoo.

I have not booked different room types, just the same standard room both days. Hilton's policy like many hotel chains is to reserve the right to delete reservations that they consider obviously 'mistakes' - but I hoped this was a reference to duplicate reservations, not back to back bookings.

When I check in, do I present both booking reference numbers? Would they be able to combine it on the spot - or would this result in their applying the higher rate for the consecutive stay (and I still can't figure out why the rate was higher for a consecutive as opposed to a disjointed stay, unless inventory is booking up and they may have to jiggle some people around a bit.) I guess what I'm asking is, do I have to 'check out' and then check back in again immediately? (I'm prepared to have to store bags with the desk in the event that I have to check out.)