A single 750ml Engel tank works well in the robbe U47 kit but the surfaced CWL is still a little low.
I think you would need both tanks to have a "scale" surfaced CWL and be able to take on enough ballast for the boat to achieve a static dive.
All Engel sytems that use two tanks have them arranged so the screw rods are angled off centre. Another way is to fit the tanks off centre (one slightly left, the other slightly right) so the rods don't touch as they motor in and out. With careful planning this leaves plenty room for batteries, radio etc,.., The big disadvantage to the Engel system is the very large WTC and the need for [B]copious amounts[B] of lead to counteract it. My U19 weighs 46 Lbs and it's no where near as big as a 1:48 scale Gato!!
A little more food for thought.