The grading (and lesson planning/assignment & test writing, though those scale anyway) would be split between the teacher giving the lesson and the one acting as a surrogate, so they'd basically still have the same load each.

Edit: probably on an alternating assignments basis rather than a "half the students' papers for you, half for me" basis, for grading consistency/fairness reasons

Edit2: Actually, for feedback-latency reasons it's probably better to split grading the other way. I wonder whether it's better to split each assignment randomly, or grade as two separate classes with shared lectures.