In case you need to assign a single lesson to several teachers at the same time, from the General Course Calendar, by locating the specific lesson already scheduled, you will be able to add one or more teachers to the current one. Then right-click on the lesson and choose the option "Edit Lesson Properties".
Then from the top of the screen, enhance the checkbox "Edit current teacher Mario Rossi" and click below on the three little stars next to the teacher edit drop-down menu to enable multiple choice.
The system will show the list of teachers who are found to be related to the subject matter of the lesson. Then enter the flag in the teacher you want to add to the current one and click the "Edit Lesson Properties" to save the operation.