I recently got the credit card and 2 free nights after first purchase. I was looking around at available hotels and it seems if I ever chose a 2 night stay the website would tell me "not available" but if I checked each of those nights individually it was available. I am happy to make 2 separate reservations for the 2 nights in the hotel (plus I will purchase a few more nights which I presume is the whole point of them offering these nights) but not sure if this is a feature or a bug.
It's a bug. Make the rez separately, then you can call in or email them and ask them to combine them, or you can do it at check-in, but doing it ahead of time usually saves some time at check-in.
-David