------ Update ------
I followed the instructions from Casa: I made A1 go into the new thermostat and then out into the blacks of B1 and C1 (see my diagram of the old setup below). Then I connected A2 directly to the whites of B2 and C2. This DOES work—the thermostat did power on but the heaters were turning on and off every 3 seconds. I have since learned that CASA DOES NOT support fan-forced heaters :(
For anyone interested, I did find a smart line-voltage thermostat that does support fan-forced heater: https://getmysa.com/.
My old thermostat provides electricity to two wall heaters with fans (I believe the heat and fans always run at the same time). I'd like to replace the thermostat with the Wifi thermostat from Casa, which only has two wires, a line and load.
As you can see, the black and white wires coming into the old thermostat are both hot. Then they go out to two lines, B and C.
I don't understand what is going on after that. One wire coming out of the thermostat (D1) connects to the black wires of B and C and the other (D2) connects to the white wires of B and C.
How do I wire the new wifi thermostat?
Here is what the old thermostat looks like when I opened the back:
And here are some instructions from Casa, but I'm not sure they apply to my situation.