Hello!
I need to hook up a baseboard heater in my bathroom
My question is . Can I wire from an existing thermostat that has only one baseboard on that circuit to another thermostat to control the baseboard in the bathroom?

NO The existing baseboard wiring and breaker is gauged for that heater only. If u add to it could blow breaker or cause fire.

Chuck

Not enough info in initial post to answer. What size of baseboard heater(watts) is currently being used on that circuit? What size (watts) is the heater you wish to add? I am assuming that this is a 20A double pole breaker with #12 wire. Where is the feed (homerun) location? In the thermostat box or within the baseboard heater?

You can add several baseboard heaters on a 20A circuit, It all depends on size of the heaters to how many can use. The size of BBheaters( in watts) depends on the size of room you want to heat. Generally it is 10Watts per/ sq ft to compute the amount of heaters required to heat a room.