I cannot reproduce this with a W Fixed 12 on top of the W Awning 1 12 (US library). Get the linework in all of the normal scenarios that I tried.
I have the fixed window above set to gang below and the awning set to gang above. After dragging them together on the exterior, the 3D is fine (attached) - although there is an extra horizontal line in the elevation linework, discussed and reported in another thread a few months ago.
The inside, of the vertically ganged windows is not correct though - the vertical casing does not meet - small gap. (Also, the default settings for the vertical casing and the stool are not particularly attractive). Second screenshot (next message).
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB