I don't know who develops most of the riffs as I imagine both Geddy and Alex generate ideas and a riff will morph to the final product.
After listening to some of the earliest Rush (Fancy Dancer, etc.) it sounded like Alex may have been the most advanced early on and I suspect most of the blues-influenced rock songs of the early albums were some of Alex's stongest contributions but it is still hard to say as that was the music of the day.
I doubt if Geddy wrote the opening blues guitar part to "The Way the Wind Blows" but he might have written the heavy riff following it.
In terms of chord progressions, jam sessions individually and combined will likely spark ideas for Geddy's melodies. In this area I'm sure both contribute about equally.