No, we can have only one actor work in a single step assignment. It can be one among
a team of workers/actors, if routed to a work queue.

It is also possible to modify security to allow work list sharing within a workgroup, not just work queue sharing. 

The Assign-Worklist GetNextWork ListView can be circumstanced for a particular workgroup to facilitate this, the query modified accordingly. The GetNextWorkCriteria Decision Tree can similarly be circumstanced and modified to add @when("CurUserHasRequiredSkills").

Locking strategy determines whether only one person, or multiple persons simultaneously, are allowed to open a case assignment, plus have perform/update/write permission.

With optimistic locking, if someone else updates a case before you while you had the assignment open, the response will tell you that the case has been updated.  You can try to save your work, but not to the case itself. Your only option afterward is to refresh the case.