Detecting Motion behind a robot
This is a basic problem of only having two sensors.  There is no way that two sensors of this simple type (some motion yes/no) can be used to work out where the motion comes from except when there is a difference between the two sensors.  Anything else is, and always will be, ambiguous.
If you think of it as a circle divided in two, you have only three possible conditions:

That is, left, right or somewhere.  Not ideal.
So you need more data to be able to make a decision.  That means more sensors.  The ideal scenario is four sensors - left, right, front and rear.  That gives you many possible scenarios, including:

As you can see from that you have a full 360° idea of where the motion is. You can even know how far to the left it is by if the front or rear is also on.  For instance, using sailing terminology, those 8 circles would mean (in order):

Motion on the left beam
Motion on the port bow
Motion dead ahead
Motion on the starboard bow
Motion on the starboard beam
Motion on the starboard quarter
Motion dead astern
Motion on the port quarter

Also the dead-ahead and dead-astern could be indicated by just the forward or rear sensor on and the left/right sensors both off.  You could even use that fact to give some indication of the distance of the motion depending on the angular coverage of the different sensors.