In map making, why is using two radars more accurate than one radar?
The accuracy of the map is a result of the accuracy of the radar measurements. In geometric (mathematical) terms, a two radar system allows you to triangulate your position - in 3 dimensions - rather than just make a single 2-D measurement.
The map maker can also use each radar as a single instrument. In this case, each radar 'checks' the measurements of the other.
But combined, the time delay between the two radar receivers can be used to gain higher resolution maps (i.e., better accuracy).
The 'tasks' of each radar in a two radar system can also differ. Whether it be the polarisation or frequency employed; or whether one radar participates as the sender (only) and the other as sole receiver (with no transmission capabilities).