Use the Multistory: Select Levels tool to create a multistory stair from an existing stair in the model.
Multistory Stairs panel
 (Multistory: Select Levels). 
  
 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). 
	 The multistory stair is created on the selected levels.