A few things to check:

• First check the boiler - is it heating water? Does your hot water tank contain hot water? If not, it could be the heating element, a fuse or part of the heater circuit.

• If you have a timer controlling the boiler, check it is working- does the relay trigger as it should?

• If that is all working but you aren't getting hot water to your rooms, next check your zone valves to see whether they open and close correctly.

• Can you hear your water pump?

• Have you tried bleeding your radiators?

• Do you have thermostats in the house? Check they are working- usually a click as they trigger at the set temperature.

It sounds like you are describing two different problems.

FIRST PROBLEM: Turning off a hot water heater.

First, there are two ways to hook up two water heaters: series and parallel.

SERIES

• In series, the water comes in to the first water heater, then passes through the second hot water heater, and finally goes to your fixtures.
• If you turn off the second water heater, then the water in the second one cools down between uses. This results in very long warmup times.
• If you want to turn one of these off, turn off the first one. (This is actually a legit way to save on energy costs if you don't need the extra hot water volume because in many places the water supply is colder than room temperature and the first tank allows the water to sit and reach room temperature before it's heated.)

PARALLEL

• In parallel, the incoming cold water is split between the two water heaters. Hot water coming out of the two heaters is combined and sent on to the fixtures.
• If you want to turn off one of the water heaters, you also have to close the valve to the inlet or the outlet. Otherwise you will just be mixing the cold water in one water heater with the hot water from the other heater.
• Leaving one heater closed off for a long time is a bad idea because the water can become stagnant and foul.

SECOND PROBLEM: Pilot light going out

Just turning off one heater should not cause the pilot light on the other to go out.