Turning off two switches at same time trips breaker

Question

I have a double light switch in my house (1 inside, 1 outside light) that will trip the main switch in the circuit breaker when both switches are turned off at the same time. Turning either one off individually works fine, and turning them both on at the same time also works, but if both lights are on and I hit both switches, it'll trip the main switch and I lose power in the whole house.

Has anyone seen this before, or can someone suggest how I can start to diagnose this? I don't understand why this could be happening.

Thanks in advance!

Edit: I'm in the UK - completely forgot about that!

Just to be specific here, the unit in question is something like this: image. The left hand switch is responsible for the outside light (and is the only switch for these lights), the right hand one is responsible for the inside lights. There is another switch for the inside lights, so this is a 3 way switch.

This is not a new installation, has never worked properly that I'm aware of, and the switch has never been replaced.

Answer 1

Tom - we need to know where on the Planet you are as Electrical Codes vary and so does the wiring.

In the USA .. simplistic description: normally your electric panel would be wired Power Line, to Main Panel ( you might have a main breaker in the panel or on the outside of the house ) your lights would be wired through a separate circuit breaker after the main breaker in that panel. Lets be clear this is NOT a Main Breaker.

So if you have a MAIN breaker that is tripping - Your circuit protection for those lights (the light circuit) is not working or it is Sized completely Wrong - hint it is probably not working - welded in the on position.

What you refer to as a double light switch is what is known as a 3-way switch arrangement. You do not specify if this is a new installation, has ever worked or if a switch has been recently replaced.

If a switch was recently replaced (my guess is yes):

I suspect the Switch; 
someone purchased and installed a normal light switch instead of the 3-way switch.

Answer 2

You have switces wired badly (you have the live on one 'inverted' contact). Just rewire the switches this way:

central should be Phase on one switch and 'lamp' from the other. Other two (deviated) contacts should be connected each other.

This is a schema about how 3-way should be wired