[limmeau]

The reason why this works is that IBM CGA/EGA/VGA map their text-mode video-RAM at segment 0xB800..0xBFFF, and in graphics-mode at 0xA000..0xAFFF. MDA and Hercules, on the other hand, map the video memory at 0xB000..0xB7FF, for reasons I never fully understood.

So, if you plug a Hercules card and a VGA card, you can have two screens. Some debuggers supported it. I never got it to work back in the day with my ET4000 and an MDA scavenged from a PC-XT, but I was just a teenager with no access to StackOverflow.