Connecting two RCD's serially
There are a few problems here.


Why are you using a faulty heater?
You should be earthing the heater metalwork, not the water.
You need to install the RCD on a separate circuit from the main fuseboard.




Update after OP's update.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Test circuit.

The standard technique for this is to add an isolation transformer. When one side of the transformer secondary is earthed (grounded) it replicates a mains supply with one conductor neutralised. Now any leakage on the test load will return to the transformer N1 terminal via the ground connection. The RCD on the primary (the office RCD) will not sense a current difference between L and N so it will not trip.

The transformer will have to be rated for maximum load, so 1.5 kVA.

The other test you should be considering doing is an insulation breakdown test using a "Meggar" or simular 500 V to 1000 V tester.