Many games, for example, Maniac Mansion, or Kissenschlacht, use two screenmodes. Oftentimes, a bitmap mode for the main screen, and a text mode for the lower part which displays scores or whatever. Considering how the raster moves across the screen, it's not so hard to implement. It's basically, set up a raster interrupt, and then change the mode in the interrupt routine. Same way you open up the lower and upper borders.
Now, with careful timing, the left and right borders may be opened up too. So I'm wondering if a similar technique can be used to change the screen mode partway across a line, so that the whole screen is divided into, say, a textmode left half and a bitmapped right half. Has this ever been done or could it be done?