Can I use a nest thermostat for two heating sources (Furnace and electric floor heating)
The Nest can be the controller but you will probably need a separate contactor /GFCI power source unless the flooring system has low voltage control.

Some heated floor systems have there own thermostat /GFCI protection, if you wanted to control the floor heat from the nest additional controls would be needed. One of the best home made systems I have seen was a 2 stage control , radiant heat takes time to heat up so an engineer I worked with asked me if I could build a control using a SCR for low power and a TRIAC for high power (SCR’s only turn on for 50% when commanded on and TRIAC’s are on for 100%)  this worked quite well at maintaining some warmth on low power and true warmth at high power, he provided the signals to fire the 2 solid state relays , I put them in a metal j box with a GFCI   And it passed inspection so I know it can be done but I think he was doing other things with the smart thermostat and using a micro controller for the control voltage.