As Martin says the car will start on petrol and automatically switch to LPG when the engine reaches a set temp. There is a switch that allows you to turn the LPG off should you wish to run on petrol.
Yes in theory you could have a full tank of each and get a great range
However LPG gives you less to the gallon. My Rover did 38mpg on petrol but 30mpg on gas, a 50 litre tank would do about 300 miles, so more regular fill ups. Also keep both tanks full is only adding weight, thus effecting performance. They advise keeping about a 1/4 - 1/3 of a tank of petrol in the car.
On the performance topic the engine still runs the same, and will give out the same bhp etc. The problem, if you see it as one, is weight. The gas system, especially the tank is heavy. The advice has always been to go one engine size up from the one you would normally have chosen if you intend to convert to lpg. I will say that none of my lpg cars have ever been slow or sluggish after the conversion, not really noticable really.
Only real differences are lack of boot space, only really an issue if the car has no spare wheel well, and cheaper fuel bills
A clubman would be good to convert as the tank could go under the "flat load floor" without really losing storage space.
