I have two baseboard heaters in the same room with one two-wire from the #2 heater to the main one. I have an external thermostat to the main heater... just one line. I hooked the heaters up but they don't seem to want to turn off. They are King ceramic baseboard. Is there a wiring diagram that shows this set up... I pulled some very old heaters out and wired the same but isn't working right. Also can you explain the value and differences of double pole and single pole?

The difference between single and double pole thermostats is not too complicated.
The single pole are usually used on 120v circuits where there is only 1 hot wire, and that is the one that is disconnected by the thermostat.
The double pole is used to disconnect BOTH hot wires in a 240V circuit.
I suspect you have wired it wrong and / or have used the wrong thermostat. There are connections for isolating one heater from the other or wiring them in tandem, as they were. Give a model number and we can look for the wiring setup.