I am using two of the electric wall heaters to heat my family room. Can these be wired together so that the one thermostat will control both of them simultaneously? Right now, they both run fine but when the temperature gets up to the thermostat setting, only one of them kicks off. Or do I need 2 themostats? Thanks

Not a problem, but the thermostat and wiring have to be sized to handle the total connected load.

Electric heaters pull a lot of current, and ganging several onto a branch circuit originally intended for a single unit might cause overloaded wiring/breaker trips.

Honeywell makes low voltage relays with internal xfmrs to control electric baseboard heat. I believe they are the R841 series. Check their website.