There are several reasons why many schools have begun to adopt a co-teaching model in their classrooms. Class size laws in many states require a lower student to teacher ratio as do many individualized education plans. Co-teaching can even result in a better teaching experience for the teachers and the students, as long as the teachers get along.
Class Size
From an administrative perspective co-teaching is extremely attractive. Many states consider two teachers in one classroom of 40 adequate for a 20 classroom size limit. This is great for schools which don’t have enough the physical space or monetary means to add extra classrooms to reduce class sizes. For teachers it means not having to face the task of supervising, teaching, and grading work for 40 students in five to six classes each day. However, there are still 40 children crammed into that one 
classroom. The ability to have those larger numbers may make administrative staff more inclined to push the upper boundaries of the class size. Having that many children in one class is going to be more difficult to teach and control, even with two teachers.
Special Education
Many individualized education plans, or IEPs, require that a student have access to a special education teacher during specific classes. By making one of the teachers in a co-teaching team a special education teacher, a school can accommodate more students in each class than if the teacher had to move between classes. For example, five students who have an additional assistance accommodation in math could all be scheduled together in the same math class where the special education teacher could co-teach with the math teacher. That special education teacher could then go to an English class and co-teach to meet the IEP specifications of another set of students the following period. The danger with this is that those special education students would be grouped together, which does not fully honor the spirit of an inclusive classroom experience.
Personalities
Many teachers have strong personalities and almost every teacher has his or her own preferred way of doing classroom management, teaching, testing, grading, and even decorating. Co-teaching offers the opportunity for great success and great failure. If the teaching styles of the two teachers mesh well together they may find the experience more relaxing and more effective for themselves and their students. If the two have completely different teaching styles and preferences it could turn into a nightmare for the staff and the students.
Have you been a member of a co-teaching team? How did it affect your overall happiness with your job and how did the children react to having two teachers in a room? Do you think this is an effective solution to classroom size regulations or IEP accommodations?