You could nest the comps in respective towel-strip comps and let it render - AE will only sample within the comp frame, after all. However, dependencies inside the 3D comps might not at all make this any faster...
Well I did a search for "after effects" +"towel-strip comp" and got nothing! You'll have to explain that a little!
The composition is all done in Particular emitting, in total, maybe 80,000 objects! Here's an early version
If I'm happy with the 'left eye' render, maybe it's easier just to render the right eye and then import, resize, and render again. Do you think that might be quicker..?
"Towel strip" is just a reference to the comp ratio, as obviously half width comps would be taller than wide. For everything else - do whatever works best for you. As long as you don't mess up alignment, pre-rendering the individual comps and 3D-ifying them after re-import should not be a problem.