In order for teachers to share classes for grading and attendance, you must give each teacher the role of "Principal". The Principal role simply allows one faculty to view/edit the classes of another.
To make a faculty a principal:
Go to the portal and to Website=>Users=>and filter on "All Faculty". Find the Teacher on the user list. Click the gear icon next to his/her name. Select Change User Type > Make Principal. Do this for both (or all) teachers that need to have Principal Authority (sharing).
Then, go to Academic > Grade Book > School Options > Principal Teachers tab. At the top left, choose the principal's name from the drop down. At the top right, choose all teachers that you would like them to have access to, including the principal's own name.