Unfortunately, the graphical capabilities of the Grid Element Marker are hard-wired into it (meaning they cannot be controlled by changing parameters). Also they are set in printed units, not model units, and therefore they become smaller or larger relative to the model as you change the scale. Also, the Grid Element has no scale-sensitive settings so they the Marker could be different sizes depending on scale.
So yes, I think you should just create a new Layer for the other scale and place additional Grid Elements for the other scale.
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-Ac27