If your gettin z-fighting I guess that means your drawing the glow polygons in step 1 as well as to the p-buffer right? I don't think you need todo that - as you'll be rendering them to the p-buffer anyway it doesn't seem needed to render them the first time. However if you do that you could create some neat effects where a blue object had a purple glow etc. You can just set a depth offset when rendering into the p-buffer (glPolygonOffset()), which should solve the z-fighting but still give you nice results.

I'm going to try both aproaches myself pretty soon so if I get it working i'll let you know.