To combine panels in a curtain wall, remove curtain grid segments. 
 Video: Place Curtain Grids 
-  Create a curtain wall with grids. 
 
- Select a curtain grid. 
 
- Click Modify | Curtain Wall Grids tab
Curtain Grid panel
Add/Remove Segments. 
 
- Click on a curtain grid segment to remove it. 
  
As you remove segments, adjacent panels join. 
  
 
 
- Click in white space in the drawing area.