that was my question.

say i am on an assignment in city A, and book a room for a week.

on wed, i have to go to city B for the night, but don't want to check out of the roon in A (say there is a pay 3, get 4 promo running there).

Or even more interesting, for diamond requalification:

I check into 3 Hotels on Thurs. Check out of 1 on Friday, 1 on Sat, and the last on Sunday. Would I get 1 + 2 + 3 nights credited?

I think you would probably be able to explain that away to customer service and get 2 stays. However, if you'd checked out you'd have 3 stays.

May be an interesting theoretical question; however, cannot think of any good explanation for this, or why you would bother. When stays are all that matters to most of us, I don't see any benefit to the multiple nights.

the only reason is if he is on the diamond challenge with 15 nights.

FWIW, I would not reccommend attempting multiple hotels in one night. It used to work, in fact back in the good old days I once actually had 3 rooms in one night all earning me FFN credit:
1) Phantom at DTC
2) Burning an FFN at hotel #2
3) Priceline stay at hotel #3

This was 5+ years back when all of these were possible. Kinda random as I forgot about the PL booking until the last minute, and also needed to redeem a pretty last minute FFN for some reason at a different hotel. But a simple incidental to the bill for Hotels #2 & #3 did the trick beautifully. -----> 1.5 FFNs earned in one day.

But alas, those were the good old days. Hyatt wisened up and put the kibbosh on all of those methods!!!

Just on a whim, I tried 2 hotels on the same day again a year or two ago during FFN (cheap rates on a holiday weekend) and the room at the 2nd hotel didn't post to my GP acct. A couple weeks later I called GP and asked for missing stay credit but they said I could not get credit at more than one hotel per night.

Moral: I would NOT recommend trying to stay at 2 hotels in one day. You'll be throwing your money away IME. Hyatt is getting too smart for us unfortunately.

Slightly OT, and this wasn't Hyatt, but I just did a mattress run at two Hiltons to finish a challenge. Reservations at two properties. Consecutive nights. Got into the first one ~2:00am. Front desk said, "I'll put you in for the Day Use rate; it's the same." A wash. I had energy for a shrug, and no more for thinking it through. (Don't know if there was a business advantage on their side?) But, obviously my check-in date became the same for both stays. Credit only given for one. It's taken some work to straighten it out. So don't let this happen.