It is legally possible to tow two trailers in this country - The fairground showmen do it often enough with lorry + trailer + caravan. It would also be possible to do it with a car/trailer/caravan, but in practice would be very difficult to achieve either safely or legally. First if all, the trailer would need to be a four wheeler drawbar type trailer - that is steerable axle at the front, fixed at the rear. The weight of the trailer and caravan still cannot exceed the maximum tow weight - so you are going to need something that can tow 3.5 tonnes. The 4 wheel trailer would be the equivalent of a car, as far as the caravan is concerned, so particularly in this situation the caravan would have to weigh no more than 85% of the trailer, and you would also have to have two trailer indicator warning lights on the dash. so to do it you would need a custom built trailer and probably add plenty of ballast to it. God Knows what would happen if the outfit started to snake!!!! So not really recomended!
Incidentally, nothing can legally out tow a Landrover - the legal limit is 3.5 tonnes, and as a Landrover can tow this, no other car can tow more. In fact on my old Range Rover, if you fitted coupled brakes instead of the overun type, you were allowed up to 4 tonnes!!