As far as I know (from experience the last time at HI Vienna), it is per stay and not per night here in Europe (maybe US/Asia too?). I did think of doing what you suggested, book two nights seprately in the same hotel, but have not tried it yet, so am not how it will work. I do find it a bit silly that you only get it per stay and not on how many nights you stay, though the points/nights needed to reach gold does take into account the number of nights stayed.