After installing two new electric baseboard heaters, which are connected to one thermostadt, the units run extremely hot such that, whrn turned on to even a very low setting on the thermostadt, the heat is still very warm to the extent that I needed to disconnect one of the units.

So what is the question?

Standard electric baseboard heaters get VERY hot. The thermostat only turns them on and off according to room temp. They do not vary the heat output of the heaters.

So, the thermostadt must be broken because it is on even when the room temp is at 70 and the setting is at 50?

Yeah, sounds like it. They are not the most reliable or durable things.