Why do both water tanks have hot inputs? Is my boiler (on-demand) feeding into my hot water heater?
If the right hand heater were supplying hot water to the left hand heater, it would be strange to hide the pipework in the wall. 

I guess it is more likely that they are feeding separate heating circuits. 

It is not unusual for the return pipe to be nearly as hot as the output line, if the system has been running for a while.

There ought to be records of the system somewhere showing how it is connected up. If the church doesn't have them, the original installers should - or at least could advise.