I have a gas combi boiler and one of the two hot taps which is fed by the boiler has stopped working. If I turn on the other hot tap then I can get hot water from both but only one is working independently. Try to turn on the other and no water at all comes out.

I'm guessing that the one tap is not flowing fast enough to signal the instant heater to turn on the flame perhaps your aerator is clogged.

Does any water come out of the defunct faucet.