I would recommend the relay. It would be automatic and require no human intervention to operate.
It may be best to use 2 relays w/ the loads paralleled to the one circ. pump, because, it is likely 2 different circuits from 2 different units.
If you knew for sure it was the 1 same circuit at both, then 1 relay would work and then you would parallel both at the relay coil. Hope this help.s