What "lock" expression are you using that only allows access for NurseA?
The expressions I can think of would either work or not allow any access when two or more nurses are listed in the same field. On the other hand, a related table can be used to list assigned nurses and the wrong expression could then permit the nurse listed in the first related record and not any of the others.
The details of how you are assigning multiple nurses will determine the details of the lock expression that will permit all assigned nurses to have access.
If you do have a single text field with a return separated list of NurseID's in it to list all assigned nurses, an expression such as:
Not IsEmpty ( FilterValues ( ListOfAssignedNursesField ; CurrentNurseID ) )
And a similar expression:
Not IsEmpty ( FilterValues ( List ( AssignedNurses::NurseID ) ; CurrentNurseID ) )
IF you use a related table to list the assigned nurses.