See what you think of how we did this job. Existing setup was 2 furnaces, 1 per floor but they had a common return. Somehow the blowers were rigged to come on together so they didn't pull backwards through the off-furnace.
On the new setup, the customer wanted variable speed furnaces. No twinning capability and wasn't sure about creative wiring so I had them replace the 1 large drop with individual ducts going up 5' to the common return hoping that it will keep return from pulling through the off-furnace. Since they will be running the blowers contantly and these don't shut off with a call for heat, should be OK. Comments?