What we sometimes did in such a case is we placed an empty opening the size of which was the same size as the two windows combined.
Then we added an Element ID to the empty Window, made the Element ID of the two Windows blank and scheduled the Empty Window.
This way sizes were good.
Not good if you want to have 3D side Views in your Schedule though.
We also turned on the Marker for the Empty Window, turned it off for the other two Window involved.
Get Archicad Tips at https://twitter.com/laszlonagy
AMD Ryzen 1700X CPU, 48 GB RAM, Nvidia GTX 1060 6GB, 500 GB NVMe SSD
2x28" (2560x1440), WIN10 PRO ENG, AC20-AC26
Loving Archicad since 1995