Once you have your groupings set up, it is time to start planning for your groups. It is very important that you plan with the teacher you are co-teaching with. This may sound like a common sense thing, but as teachers, we are very busy, and many times we do not have time to sit down with others to really plan. It is often a skipped step.
It is best to have a set time each week to plan together to make sure that you are on the same page for the upcoming week and month. This is something that I haven't done as well as possible, and I know that true co-teaching has been better when we have taken the time to plan together.
When team teaching, you really need to know the exact direction that you will go in. For the first few times, it may be beneficial to write a basic script to follow to help get into the groove. Once you are used to team teaching, it will come must easier.
For subjects such as Math, there are a variety of methods to complete different problems. Plan which ways you will show them. Maybe each teacher will show a method, but these need to be planned and agreed upon. The worst thing is to be on different pages and not agree with each other in front of the students.
Do you have a co-planning time? How is it structured?