For standard baseboard wring on the right or left 120V or 240V supply.
If you are going to install a new baseboard heater, this article will be helpful for you.
This diagram from the Baseboard Owner's Guide, shows how to you can wire multiple heaters to one thermostat — if you're using volt.
This is also caused by a connection issue.
Important: Modifying existing electrical circuits or installing additional electrical wiring should be done according to local and National Electrical Codes, with a permit and be inspected.
I will be running new wire and putting them on the same circuit.