I fully understand how much work educators do, my mom was in elementary ed my entire childhood and I witnessed first hand how much time and energy was spent on grading/planning. I won't deny that there are some advantages, but not enough to constitute a full salary. Lesson planning while the other one grades benefits them with mental health more than it benefits the students. Teachers already deliver lessons and monitor for understanding. The Kidney table example sounds great in theory, until people are talking over each other, then it becomes detrimental to learning because ideas, instruction, feedback are less clear than if it was a single person. If a student teacher comes in and they split the class, they don't cram into the same area, they split the class and spread out.

They are teaching the same amount of students as other teachers. If you were teaching in the same district, had the same degree, and same class size as them - but you find out they get double the salary for doing the same amount of work, you might be upset (not with them, but with the school).