How to connect two solar water heaters in series

Your idea is good. Simply connect the water outlets of the two water heaters together, then use a tee to open a hole in the connecting pipe to bring hot water to your home. But I don't know the heat. collection capacity of your water heater and whether the thermal insulation capacity meets the standards, otherwise, it cannot be used for heating. Ordinary radiators cannot be used for solar heating. Forced cooling fan radiators or underfloor heating should be used. Solar energy should be determined based on the heating area; the insulation capacity of the room, the power of the water pump and the flow rate must also be taken into account.