Using a doublepole thermostat to control 2 separate electric 240V baseboard heaters. Does each baseboard circuit run to thermostat,or does first unit wire to second and then to thermostat?

Ou can do it either way. If both cables from each heater come back to the stat, then both get connected to the LOAD side of the stat.

Or one cable can leave the LOAD side of the stat to the first heater, then then continue onto the next heater.