Turning off two switches at same time trips breaker
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.