James
When you speak of "Diamond privileges" do you mean lounge access? I think that those are vested in the holder of the Diamond card plus one guest, and not to anybody who is on the same booking. I have had experience of this being applied flexibly, however. If you are given an upgrade on both rooms to the Exec floors the problem then goes away.
Purely practically the HHonors site would, I think, allow you to make two bookings at the same property of different durations and give alternative traveller name for one of the bookings. From the booking pages "Your name will be associated to all rooms in the reservation. If needed, you can associate one additional guest name per room to allow them to check-in separate from your arrival.
Hope this helps
Paul
We can get better, because we're not dead yet