Answer:
Well it all depends on the situation. For example if the main pastor of the church is sick many time's the co-pastor of the church will preach the sermon. And some times the co-pastor if some one needs help say like there is some one who needs prayer and the main preacher is in the middle of the sermon a co-pastor may ask if he or she can help pray in some way or say the main pastor is already helping some one pray the co-pastor can also help another person that is need or a prayer partner.