How do you wire a 240V switch so that you can run an electric heater in the up position and a table saw in the down position?

If it is a simple two-pole switch marked on/off, you cannot create this situation. You could accomplish your intention with the use of a double-pole, double throw switch, center off [which would have six terminals]! Since you are controlling a saw, you would need a horsepower rated switch. Guessing that this is a domestic application, safety would suggest using a locking type switch. [But, then, I wouldn't put a table saw where I had electric heat either...] <><><> For a 220 run, the answer for those who want to know, is a 30amp DPDT(Double Pole/Double Throw), maintained contact switch. Leviton is one manufacturer who makes this switch although you can expect to spend anywhere between $60 to $90. IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.

If you do this work yourself, always turn off the power

at the breaker box/fuse panel BEFORE you attempt to do any work AND

always use an electrician's test meter having metal-tipped probes

(not a simple proximity voltage indicator)

to insure the circuit is, in fact, de-energized.