Scrolling Theory

Ok, so we have two video screens, one which will be located in bank 2 (paged in at #8000) and the other in bank 3 (#c000) which will contain roughly the same but shifted left by one pixel. And we'll scroll these using a combination of CRTC register 3 (for a 2 pixel shift) and standard address offsetting using CRTC registers 12 and 13 to perform the main scroll.

So, what does that look like in pseudo code?

Frame 1
reset CRTC register 3
select the screen at #8000
draw a shifted column of bytes at #c000 + offset + screen width

Frame 2
select the screen at #c000
draw an unshifted column at #8000 + offset + screen width

Frame 3
move the display using CRTC register 3
select the screen at #8000
draw a shifted column of bytes at #c001 + offset + screen width

Frame 4
select the screen at #c000
draw an unshifted column at #8001 + offset + screen width

Frame 5
Increment the hardware offset
reset CRTC register 3
select the screen at #8000
draw a shifted column of bytes at #c000 + offset + screen width

and then repeat from Frame 2 onwards

Note that in each case we're always writing to the unselected video memory, so that we don't need to worry about tearing.