Honeywell C wire on a dual transformer system

Currently I have dual transformer system
From heat >> RH & W1
From cool >> Y1, Rc, G

Honeywell provides a C wire adapter and also mentioned turning an unused wire into a C-wire. I do have an unused wire from the cool side. Mentioned in the manual is all about joining the adapter OR if there is an extra wire to furnace. My furnace wire up to the thermostat is only 2 wires.

Is it possible to use the cool transformer for the extra wire? Since I do have 2 unused wires

Thanks!