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
AC 28 USA and earlier • macOS Sequoia 15.3, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB
One of the forum moderators