Why hook up two thermostats to one gas furnace?

My "furnace" is a boiler with one zone circulating the entire house.

I have a wood burning fire place on the main floor. The thermostat is also on the main floor.

When I burn the fire place, the main floor heats up and the lower floor gets colder than crap.

I want to install a second thermostat on the lower floor to keep the boiler @ 70 degrees....to keep that lower floor at 70 degrees....inspite the fact that the upper floor, with the fireplace burning, raises the temp on the main floor up to as much as 80 degrees!

Yeah, yeah, why burn the fire place and the boiler at the same time. Answer, to keep the lower floor warm!

Can't run duct work from the main floor fire place to the lower floor in a finished house!