Shower and undersink water heater.

I have a property that has 6mm mains swa cable coming into the consumer unit via an RCD isolation switch. The home has a 7.2kw electric shower and I wanted to add a 7kw under sink water heater. So as to prevent overheating of the main cable and tripping of the main incoming fuse which is just 40 amps, could I put a 32 Amp 3 Pole Change Over Switch ahead of the two six mm cable wires which would be serving the shower and the water heater.  Therefore negating overloading the main wire coming into the home by allowing only one of the appliances to be used at a time.