Quick question: if I change the screen-map address in the middle of a frame, will the scanlines rendered after the switch start from the top of the new map, or the "correct" position in the new map?
The scan lines will start from the "correct" position (as you say); it will not start at the top. However I must clarify a bit...
In bitmap mode, "screen-map address" might be interpreted as the bitmap data (pixel data) or as color data. With bitmap data, the change happens instantly... in the middle of a scanline if you wish.
But for color data (of bitmap), or for normal text mode, the change will not happen until the next "bad line." By default this occurs every 8 rasters (at the first/top raster of each character row). Of course bitmaps don't have character rows, but the color data of a bitmap is fetched like text/character rows.
In character mode you can force a change sooner than every 8 rasters by either changing the charset-address (not screen map address) or by playing with the Y-Scroll bits... too complicated to fully describe now, I refer you to Commodore Hacking articles for more info.
Interesting. I'm not really looking to do anything fancy, though; I'm just trying to figure out how to arrange video memory in a potential project. Both screens are character-mode (although the top one is multicolor,) so I wanted to know if I'd need to keep the lower screen in a different character-map area. Looks like not, though!