You must be doing something unusual because it works for me just fine.
Are you sure you have placed the Windows using the Corner Wall Tool (available in the More group of the Toolbox)? It should place both Window of the Corner Window when you place it near the Corner.
Also, when changing parameters of one Window (including Materials), the corresponding parameters of the other should also change. The only exception to this rule is horizontal sizes as the two component Windows can be of different length.
Loving Archicad since 1995 - Find Archicad Tips at x.com/laszlonagy
AMD Ryzen9 5900X CPU, 64 GB RAM 3600 MHz, Nvidia GTX 1060 6GB, 500 GB NVMe SSD
2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac28