A shift is a task that somebody has to perform on particular days. Each shift can be performed by one person on each day that the shift occurs.
A shift does not have to be an actual period of work, it could be a responsibility such as "Meeting Chairman" or "Contact Point".
Other examples of shifts are:
If you need to model an activity which requires two people then simply create two shifts, such as "SHO on Call #1" and "SHO on Call #2".