I think it looks great. A clever way of fitting inside the 80m box while doubling capacity.
As the rudders at 'J' are split, I think they would work just fine for yaw control, using asymmetric drag.
One concern could be the position if the engines behind the front wing. At moderate angles of attack they may encounter disturbed air.
Why don't you send your ideas to the Royal Aeronautical Society? I'm sure they would be interested and have some well-qualified people to give constructive feedback, instead of the nit-picking you may encounter here.