Which Transformer for 220v to 24VAC for Heating Smart Thermostat
My plan worked perfectly and I called Emerson Sensi customer service to check that it will not cause any issues and the confirmed.
The 2 wires to the 24VAC relay that control the baseboard get connected to the Rh and W terminals on the thermostat.
I cut the jumper between Rh and Rc to ensure I wouldn't cause a short by adding another power source
I used an external 24VAC transformer, connected to Rc and C to provide power to the thermostat.
In essence this is not a parallel circuit situation, but 2 independent circuits. 
One circuit from the external transformer provides power to the thermostat. The other circuit controls heat activation.