Its not uncommon to want to edit a shift that normally has one user, and split that shift across two users.
1. enter edit mode on the call screen
2. click on the pencil next to the shift you want to split - the popop menu will appear.
3. Click the "split" icon.
3. The dialog allows you to choose the time that the split will occur, and which user will start or end the shift. One of the two users must be the original user.
After you've chosen the time and users, click "submit". Both users will be notified.