How to prevent spdt shoot through?
Your application seems unsafe for a couple reasons:


There is no guaranteed isolation between the contacts even if they configured to break-before-make. In other words, from safety point of view, there is no guarantee that the MPPT controller will be isolated from the AC network. From a product/earth safety perspective this solution is dangerous.
Even if the AC network is actually disconnected from the network, the boiler and connected wires have their own internal inductance, and once you open the relay, it will cause a spark (depending when the current crosses zero).
If solar grid is wired in a SELV network, you are causing an unwanted risk by sharing AC with SELV on the same contacts of the relay.


The solution would be either:


Use an isolating transformer from AC to SELV to power the boiler
Use an inverter next to the MPPT controller
Use a bolier that is designed for the purpose


As already pointed out, since the power to the boiler resistance is V^2/R, you will get an heating effect that is much lower (and maybe negligible).