Boulton-Paul.

But if you are stuck with the MK1 eyeball aren`t 4 better than 2?[Spit,Hurry etc]

In the context of the Defiant - no, anymore than two chaps running a three-legged race have an advantage over a single runner. Four eyes and a plough horse present no advantage over two eyes and a thoroughbred.

The Defiant was, in concept, more akin to the BE.2c - an airplane where the pilot had no armament of his own and had to depend upon his gunner (and try to think in unison with his gunner). The best two-seaters of 1914-1918 gave both men a weapon, enabling the pilot to maneouvre, attack and shoot while the gunner covered his tail. Attempts to replicate this concept in the Second World War failed (the Bf.110 was a dud in its designed role). With the failure of the Defiant (and the Blackburn Roc), the succesful two-seater fighters became those that either took the guns away from the gunner (as in the Bf.110) or never gave him guns in the first place (the Mosquito), replacing the gunner with either radar or long-range navigational duties.