Exactly the same position in our school. One full time teacher split over two different classes (and year groups). I can't see the point myself. I like sharing with who also teaches my class, but when they are in the other class for things like parents evening, it can get difficult. Not looking forward to writing reports. Legally you are stuck where the head choses to deploy you. If it is that stressful then you need to think about looking elsewhere for a more straightforward job.