I am officially daunted. As a "technical" type, I often really underestimate how difficult projects are.
The current system is set up as two zones (upstairs/downstairs), since that was the easiest way of connecting the pipes. I'll abandon the second boiler and look into purchasing an economizer.
Moving forward. I can see where to adjust the temperature setting on the aquastat, and it definitely was at 180*. I've turned that down to 150 for now. You're suggesting I continue to turn it down as long as the return doesn't fall under 130? Would I be better off padding the return temperature? There is definitely no bypass of any sort in the system - it's boiler -> zone valves -> radiators plumbed in parallel -> boiler.
Thanks again for everyone's help!