I feel your pain. A couple of years ago I was writing code to drive an Optrex F-51405GNB-LW-ANN, and it had just such an arrangement with two completely separate driver subsystems, splitting the panel right down the middle, and I went through all kinds of contortions to get images to scroll smoothly across the boundary.
On that one, one CS line was high for the left, low for the right.
On yours, I don't see that there two controllers. There's two column drivers, yeah, but those don't cause problems, and anyway you know you can write all the way across - so THAT ain't it.
According to that diagram, you have just the one ST7920 doing all 64 rows high. Did you get the spec sheet for the ST7920? This will tell you a lot more than the crappy data from the LCD manufacturer.