Say, did you try the crawl method? (Sorry, couldn't resist...j/k.)
If you can't put the sub in its ideal "crawl" location, then you should just move it around until it presents its best bass at the listening position. First try corners and also in a nearfield position (behind the couch perhaps).
Yes, a second sub (not co-located) can help minimize the nulls and help flatten the bass frequency response.