Yes. More than one teacher can work on a specific plan at the same time. As each teacher edits or adds to the plan, their work is saved to all the collaborators plans as well as their own. (The changes become visible on collaborators’ plans once they refresh their screen.) 

Teachers can see who else is working on the plan at the same time. If more than one teacher happens to work in the same text box at the same time, the first change is saved.