I don’t know of any study which looks at this. However, isn’t the point of Z1/2 to cause adaptations through acute volume? The exercise has to causes enough stress to force change.
If A is doing 7x2hr and B is doing 3x4.5hr, the winner will be the highest chronic load of the highest acute volume (Eisenhower Matrix!). They will both eventually plateau; could A reach the same level as B given more time (under the A protocol)? 