Effective teaching has in mind the various needs of the students, being conscious of varying levels of understanding, pronunciation, speaking, writing and listening skills, and the needs for control of student behavior so that all have the opportunity to learn.
When one is teaching, the other needs to be focused on the students' teachability, and give input when necessary. Both teachers need to be very present to the students for control of the classroom and thus, effective teaching. English should be spoken for every teaching component and activity, offering translation when necessary.
If the Korean English teachers model speaking English for the students and make mistakes, the students will know it's okay to make mistakes, as long as they TRY to speak/learn English.
For co-teaching to be fully effective, there needs to be open communication, use of humor, mutual respect, flexible equality, complementation of each other’s strengths, and collaborating on teaching roles.