At this moment- co-teaching is not a feature. It's been asked, so it's definitely on our radar. The solution to this, since you are an admin, is to go to: https://www.tynker.com/myclasses/admin/#/sa/teachers
Click on any teacher's email, and you can simply log in as them. So the class will be under their name, but you'll have access to that teacher account. Or create a temp teacher account for this purpose that you can both log into.