This is a common word that is not used very often in music, it refers to two lead instruments being played simustaniously together with niether overlapping the other, as a result creating a nice fusional smooth yet intrigueing sound when heard. It is used rarely and is many times unkown by muscians.
If you listen to the last ~2-3.5minutes of freebird (by Lynard skynard) you will hear a poly playing at some point.
Guy #1:Omg, remember that time we were playing the poly in Gh2 for free bird?
Guy2#:Yeah it was pretty sick, but ive heard better from other artists.