You simply want to throw a switch and have the compressor power up, and have
power cut off to the heater?
Two ways to do it.
Get a double pole double throw switch, and hook it up to the two units.
Thrown one way the compressor will have power, the other the heater. In the
middle neither. It is like a 3way switch, only for 240v.
The more elegant way is to use a 240v relay with normally on contacts. When
the compressor switch is off, the relay will allow power to the heater.
When the switch is on, the relay will open shutting off the heater.
The DPDT switch is certainly easier, but it may require you to move some
wiring around. With the relay everything can stay where it is, with a small
cable connecting them.