We had experience with both, but not in the same year. My son loved the job sharing teachers. They were both excellent and parents gave good reviews because the teachers were allowed to continue to share up until one decided to stay home after the birth of their second child.
The 1st/2nd split thing. I was in a split class and loved it. My son was in a split class and really didn't care for it much. To be honest, it has more to do with how capable the teacher is in providing instruction to 2 different grades. She had a lot of volunteers and still did had trouble with separating the work. She ended up giving both grades the same spelling and writing homework because it was easy for her, but honestly it wasn't appropriate. The reading and math groups were done by capability, so it was fine that 1st and 2nd were mixed. The school did in the end provide an aide to help with teaching more to each grade.
That being said, when I was in a split class, my teacher was capable. The same school where my son had issues had a combo class last year with a different teacher and the results were far better.
Good luck and if you have an reservations, it's best to raise them with the principal sooner rather than later.