I'll try to be simple.
Lets say you dont have anything on both disks, they are both empty, you load up the boot usb drive and install the custom iso,it installs and everything is fine,if you try to install a second windows, a normal one , it installs yeh, but as soon as restarts the pc it goes for the recovery screen, even with bootmenu display enabled, you can't choose the windows you want to load up, just instantly goes for the recovery screen. tried everything possible tried installing either way, first installing the normal iso and then the custom one and recovery screen all over again