I can do this with a Nest thermostat but the Nest uses a cheap chip instead of an actually relay between the Rc and Rh and it will burn out during installation if you do not shut the power off to both units before hooking it. Very Annoying. I have not had an issue with old Honeywell thermostats doing this.
-- Thanks, Anil A. Apple Communication LLC. www.apple-link.com Pro-AV - Pro Lighting - Networking - Security Cameras - Home Theater For all your low voltage cabling needs