Is there a way to permit to enter a value for a global rotation somewhere, if not WHY???
Just to clarify a little more.
Regardless Local, or Global, individual object is always rotated around its Pivot.
When 2, or more objects are selected, the resulted Pivot is located at the medium between all selected objects Pivots.
You CAN also modify individual pivot for objects and place it precisely at the medium between 2 object (play with Edit Pivot).
Otherwise as Gerry pointed attach them to a dummy...