Can a 110v spa pump curcuit control a 220v spa heater that is on it's own 220v circuit?
I'd guess your best bet would be a relay.

You could connect a small power supply (transformer + rectifier + capacitor) across the pump supply, so that when the pump is on the power supply gives say 12V.  That 12V could then power the coil of a relay which controls the 220V circuit.

You could build that circuit for a few dollars.

Alternately you could rectify the 110V and use that to power a relay with a 110V coil, but that could be dangerous and the capacitor to smooth the voltage (and stop the relay buzzing) would be quite expensive at 110V+.

(I have no idea what an SPA pump or SPA heater is, but I assume by the voltages mentioned, that they operate off AC voltage.)