I have a 20a double pole line with one 1500 watt baseboard heater ,#12 wire on a circuit.
I need to put heat in my bathroom ,its 5x8
I have no more room to put breakers in my panel
can I connect to the circuit mentioned above to install another thermostat and heater for my bathroom

Yes, you have 1500w heater on a 20A circuit. On this circuit you can use 3800W max. There for you have 2300W remaining on this circuit.

Make sure for certain of the load on that circuit before altering.