Want to add a WIFI thermostat, but no 'C' wire... only 2 wires to each thermo
The "C" in C-Wire stands for "Common", so what you propose should work.  The 24VAC transformer has two wires coming from it.  One of the wires goes to the thermostat and that is what the thermostat actually switches on and off.  The other wire is typically labeled as "common" and it goes directly to the relay(s) that the thermostat controls.

To say it another way... older thermostats didn't need power to run, so only having one wire from the transformer was fine.  For a new powered thermostat, you need both wires from the transformer, and that's all the C-wire is - the "other" wire from the transformer.