Two supersonic planes. Do they hear each others sonic boom? Plane A is traveling at Mach 2 and is over taken by plane B traveling at Mach 3.  
Does plane A hear the sonic boom from plan B?
If so when?
Does plane B hear the sonic boom from the slower Plane A?
If so when?   
My guess that A hears the boom from B when the cone from plane B overtakes A.    
Plane B is way more confusing for me as it starts in the cone of plane A and then exists the cone of plane A prior to overtaking plane A.
WIKI sonic boom
 A: You hear the boom when you and the cone overlap. It doesn't matter whether you move "into" the cone, or "out of" it - there will be a sharp transition in pressure. Maybe plane B hears a "moob cinos". It will still be loud.
A: I spoke with fighter pilot and instructor with a degree in aeronautic engineering.   
Yes both planes produce a sonic boom.  But how the boom impacts you is dependent on your velocity relative to to the boom.  The boom is a single percussion wave.  If you are traveling with it you basically slow it down - it takes you longer to get from one end to the other.  It is like a boxer taking a punch.  You can even take it out of audible range of a human ear.  Even a single percussion wave has a frequency. The trailing plane entering and exiting the cone is aware as the pitot tube velocity goes out of whack but it is not that big a deal.
So for my direct question of do you "hear it".  If it get taken out range of your ear then you don't hear it.  
As for approaching the wave still has the same amount of energy but you experience it faster.  It could get taken out of the frequency range of the human ear on the high side but you would be more aware as you took the hit in a shorter period of time.
