If you are using a cooker socket it needs to go on the LHS or use an RCBO on the RHS as the socket requires RCD protection.
FYI it is not a split load board, it is a 16th edition board.
It would be quite simple to do with a relay off the first stat that when satisfied then closes a contactor to run the second heater.If the first then calls again the relay would open reverting it back to the first heater.
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