Use the Multistory: Select Levels tool to create a multistory stair from an existing stair in the model.
 Multistory Stairs panel
Multistory Stairs panel
 (Multistory: Select Levels).
 (Multistory: Select Levels). 
   is selected on the ribbon.
 is selected on the ribbon. 
  For example, use a selection box or press and hold Ctrl while clicking the levels. Press Shift and click a level to deselect it.
The selected levels highlight, but you will not see the stair extended until you click Finish.
 (Finish).
 (Finish). 
	 The multistory stair is created on the selected levels.