Making sure only one of electric shower or instantaneous hot water heater can operate at once

I'm completing the renovation of an annex. The annex shares the mains supply with the main house which has a single phase connection rated as max 90A. I'd like to fit an electric shower (9.8kW) and an instantaneous electric water heater (9.5kW) into the annex; giving a combined draw of ~80A. Clearly using any other power hungry device in either the annex or main house (eg kettle, oven etc) would then take me over 90A. I thought that a possible solution is to only allow the instantaneous water heater to draw power if the shower isn't operating. Is anyone aware of any product which would cut power to the water heater if the shower is in use so that the max draw would be reduced or any other cunning solution (other than ripping out the shower and water heater)