Always difficult to speak in absolutes when it comes to Disney transportation, you can encounter an exception to an experience someone had the day or hour before or unusual circumstances (both positive and negative) at any given moment on any given Disney day.
Generally speaking they always share on the return.
They usually share heading to the parks as well although it's not out of the question for some single resort service during morning rush periods. My personal experience with this has varied - I've been on a full Poly bus that opts to head to direct to the park and also a full Poly bus that makes a totally useless stop at GF.

My sense is they do try to dispatch buses to offset the GF build-up, but inevitably someone will find themselves on the losing end there as nothing with bus routes/capacity happens at the snap of a finger.
Just a reminder for less experienced guests, the only bus service offered at either resort is DHS, DAK, and DS.
Personally, while you can occasionally find yourself on a full bus to or from either resort or unfavorable delays for whatever reason, the majority of my Poly/GF bus experiences on average have been pretty efficient and lightly utilized, frankly.