How do you Connect Two Wall Heaters Two 20A breakers and One Thermostat?
You can't. Good news is, you can use normal 'stats like the Nest

Your system uses line-voltage thermostats where the 'stat itself switches high-amperage 240V.  Generally you can't control 2 separate circuits worth of heaters from one such thermostat, because most 2-pole 'stats only switch one pole thermostatically and the other pole is only turned off at the bottom (hard off) position. Your old thermostat is a 2-pole which switches both poles thermostatically, which makes it unique.  You'll be hard pressed to find another one like it.  No smart thermostats exist which are capable of it. 

You must fully separate these 2 circuits, but that is impossible with the readily available stock of thermostats.  Therefore we are left with junking line-voltage control entirely, and going with 24V style 'stats.  The 24V control voltage can freely visit both circuits' heaters without restriction. 

In this case you have a 24V transformer feeding the "R" and "C" wires.  A relay on each heater is activated by the "W" and "C" wires.  And a thermostat which uses "R" and "W" to call for heat, and "C" to power itself.   Very standard stuff.   Standard color codes are red for R, white for W, and blue for C. 

Heaters usually have extra space for this sort of apparatus.  Each heater gets a relay (unless there's an advantage to putting the relays near the breaker panel), and the transformer can go near a relay.