I have a dual transformer system - one for heat, one for cool. The
thermostat I'm replacing has an Rc and an Rh connection. The new
thermostat has a single R connection. The transformers are in phase
with each other, however, the heat one puts out 24vac, while the
cooling one is at 26.7vac. Can I connect them both to the R connector
on the new thermostat, or will that not work?

Here's what I'm trying to do: I'm replacing a White-Rodgers thermostat
with a wireless one from Carrier. We recently had AC retrofitted to
our house. The house is a small two story cape, and because of the
size and space constraints, the AC is a single zone. The thermostat is
on the ground floor, and my wife, who is handicapped, would like to be
able to adjust the temperature while upstairs. It's very difficult for
her to make the trip down the stairs too many times.

labman

I don't have much experience with dual transformer installations. I am just a home owner who has ratted out some of this stuff my self and am passing on what I know. I think it should be OK as long as the transformers are in phase. The voltage difference is between the hot and common. As long as the commons are not connected, the heat transformer should never see the higher voltage.