This website has a plethora of resources on an instructor using co-teaching with a student teacher. The method that is most reliable is the first method - one teach; one support. Additionally, there is a page of communication to be successful.
Slides 23-24, 26, and 27-28, 30 offer information on how to best incorporate a tutor / SI leader into your class. One teach; one observe and one teach; one assist are the two strategies that would allow successful implementation of tutors / SI leaders into the classroom.