Dear Mad,
I'm not sure I fully understand your problem, but have you tried Window Assemblies as another way of creating a double-sash window? This gives great flexibility with regards to frames, mullions and the sashes themselves in that: 1) The outside frames can be dimensioned separately from the mullion 2) the mullion can dimensioned separately from the frames 3) the widow sashes are independent from the frame and mullions. Additionally, you can apply a frame or mullion profile so that they can accurately represent the actual look of the window (as far as you want to go). The fill regions (2 in this case) bounded by the frames and mullion are set to use a window style (i.e. double hung window without a frame as the style) and as you change the assembly dimensions, the sashes will change accordingly.
