First, ensure that the co-teacher is listed as a teacher in the relevant classes in Google Classroom.
The co-teacher should then created an ASSISTments account with their Google Classroom login credentials.
Next, they should attempt to assign any problem set and, when the reach the Assign options, select Google Classroom, click Import/Sync
Next the co-teacher should choose Synchronize.
Once this is done, the co-teacher will have the ability to assign to and review reports for your shared classes.