Help with Nulls

[DjGuinness]

So I did a subwoofer crawl, placing my sub at my seating position and going around the room, staying low and looking for peaks.

For the most part, the front 2 corners did well, with the right corner being slightly smoother. Smooth all the way down to below 40 Hz. Playing a 30 Hz tone was near inaudible in both front corners, and as is 30 Hz cannot be heard where I sit. I can push it till it sounds like the windows are going to break, but can't hear anything.

However in the back two corners, 30hz is heard deep and clear, while 40hz doesn't seem to have the same smoothness as the front corners.

Is it normal to have a peak for a certain frequency range in one area, while another range is completely nulled?

Is that the big advantage of having two subs? Would you all suggest I look into getting another sub for the back corner as well? As it stands right now with one sub, where should be it's permanent spot, front or back corner?