Also, if the yellow areas are far in the background, you can simply ignore that since it's not important for calibration or tracking. With Kinect, getting an accurate read on the ground plane and figure reconstruction is way more important than the background.
In general, to get 180 degree tracking, the sensors should be not be facing each other at exactly 180 degree because of interference. As VMaslov points out, you should have them angled slightly away from each other, so they should be positioned at less than 180. IMO, I would go a bit less than 180 to minimize the interference, especially if the room is small.
FYI, most of the time I shoot using the 90 configuration. This requires less space and it's still able to capture many 360 degree motions reliably. Here's an example using the 90 degree configuration:
Sister's Mocap Test.
G.