Material between mulled windows: making it the same as the frames

[GeneDavis 72]

I ganged these windows and blocked them, but used the factory-mull spacer distance (1/2") Pella will use when making them.  See the picture.

 

We've no way to control the material for such a mull-joint part, with Chief assigning it the same material as the casings.  

 

I know I can make it go away by unblocking the windows and resizing so there is no more half-inch space, but I'm a purist when it comes to millwork, and want the windows to be true-size as they will be ordered.

 

Others rant here about insignificant elements of 3D, so consider this mine for the day.  I'll go to the Suggestions section and ask for a future mod.  And for this job, we'll just live with it.