I am a high school English teacher. I currently teach six sections of 30-35 students. I already spend my nights and weekends lesson planning/assessing writing, and you want to double my student load? Fuck off. I’d quit tomorrow.
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.
When I was a TA we’d split the stack of paper tests evenly and rotate them every few days to grade them in parallel. It’s even easier to do the splitting if tests are submitted online.
And with two teachers, you'd also be able to have each write half the questions, and split the grading that way.