Mistakes can and do happen. So is it possible to receive extra night credits for multiple rooms? As leeky points out, technically, the member should be the one paying for those rooms. But again, you might might luck out and get the extra points. Just don't assume it will happen and if it does, great. If it doesn't happen, you weren't expecting it anyway.

Note that over the years, I have seen "mistakes" occur more often in certain situations over others. For example, I've been more likely to receive extra night credits when booking multiple rooms at non-FS hotels. FS hotels seem to make that mistake less often.