Is the thermostat in the end of the heater or on the wall?
Are they 120 volts or 240 volts? What is the breaker that feeds them? Are they both on the same breaker?
The thermostat is on the wall. it is on a 240 breaker and the thermostat controls both heaters.
Go to Home Depot and ask for a wall 2 pole line voltage thermostat for baseboard heaters. Make sure it is rated for the same or greater amperage than the breaker that feeds the 2 heaters. These thermostats are universal. Any one will work. The brand does not matter.