It has been proven time and time again that smaller class sizes produce higher achieving students. Through a combination of more personal instruction on behalf of the teacher, a generally more inclusive classroom environment, and more resources per child. As for the two teachers aspect of it, the classroom isn't a late night talk show, having two teachers in a classroom is actually a widely used strategy in rooms where students with special needs (i.E. BIPs, IEPs, or 504s) make up too large a percentage of the classroom population. With extra teachers the one on one instruction is even more evident. Overall, two teachers and twenty students would be an ideal classroom environment.
The effectiveness of student's studying is not determined by the number of teacher. How actively they accept what teacher's teaching and how hard they study for themselves is more important. Decreased number of the students in a class can be more helpful to students to learn, but in my opinion, 2 teachers in a class is unreasonable.
In my opinion, twenty is a small number. When there are 2 to a group, opinions and strategies may clash, leaving the students confused and unable to learn efficiently because different teachers may teach differently and might have two different opinions. Teachers may also argue among themselves and "fight" for the "limelight" and it is very degrading towards one if the students have more preference towards one teacher compared to the other. I totally disagree that all classes should have 20 to a ratio of 2, unless the class is divided into 2 and each group is allocated to one teacher.