Can a C wire be used for two thermostats?
I think NO. The C-wire connects to the transformers on the individual units.  It's not like a common neutral or ground wire. You'll need to get a C-wire to both furnaces as there is no real electrical connection between the two on the low voltage side.  There might be other solutions that the "big three" might be able to come up with, but I think your stuck with getting a C wire to each furnace independently.