You mighjt be able to add two heaters together but only if the total wattage for both does not exceed the maximum for the circuit. If you can’t figure out the calculations to determine this, hire an electrician.
Also, it is not allowable by most codes to run electric heat under existing outlets. If adding an additional length of base heat goes under an outlet you will breate a fire hazard for anything that is plugged in above it. There are ways to still wire two units together without them physically touching, but that would involve running wire inside the wall between the two.
If there is no code reason why you cannot join these two units together, you can actually wire the heat units either way:
-power to the unit first, then to the thermostat
-power to the thermostat then to the units.
It is simpler for most to wire to thermostat first in most cases.