If you open the elevation view of one of those window from the Schedule you can copy and paste the fills and lines of the other one into that view so it will look like one window.
Then don't show the other window in the schedule - or show them both and note that the two windows are to be mulled as shown in the Schedule view of the other.
What about mulling them together (ganging in ArchiCAD speak) and saving it as a new Window Object ? Geometrically that works, I don't know if all the window settings you would want could be preserved that way.
ArchiCAD 25 7000 USA - Windows 10 Pro 64x - Dell 7720 64 GB 2400MHz ECC - Xeon E3 1535M v6 4.20GHz - (2) 1TB M.2 PCIe Class 50 SSD's - 17.3" UHD IPS (3840x2160) - Nvidia Quadro P5000 16GB GDDR5 - Maxwell Studio/Render 5.2.1.49- Multilight 2 - Adobe Acrobat Pro - ArchiCAD 6 -25