Yes you can easily create a larger display that is 256x64 instead of 128x128 if that works better.
It is just a matter of how you define the geometry and the chip selects.

It shouldn't be that difficult as you won't have to modify any code and
the the master ks0108 panel config files already have the necessary support for this.

There are 2 ways this can be done.
You can either use a autoconfig panel config file and then use pin configuration files.
OR you can use a manual panel config file.
When using a manual panel config file everything is in that one file including all the pins.
A manual panel config file will not select different pins depending on the board type.
Its pins are hard-coded/locked down.

The changes for either are basically the same.

====================
For a autoconfig panel config file

You can take a look at the ks0108-192x64_panel.h configuration file
to get an idea how this would work.

You can modify the autoconfig ks0108-Panel.h header file,
but I'd create a new header file by copying one of those
and use that one for this instead.

Then,
You will want to modify the DISPLAY_WIDTH define to 256

Then change the NBR_CHIP_SELECT_PINS to 4
Also (just for clarity if you run diags) alter the glcd_PanelConfigName string.
(it isn't really needed but might help avoid any confusion later)

This assumes you are going to use 2 glcd modules each with 2 chip selects.
(which is normal unless you want to add a decode chip so you can avoid having use 2 extra AVR pins)

You will then need to modify the master glcd_Config.h file to include
your new configuration file vs the original ks0108-Panel.h file.

==================
For a manual panel config file

Take a look at ks0108_Manual_Config.h
The same changes mentioned above apply.
The only difference is that the manual config file also has
the pins defined inside it vs the autoconfig panel config file references pin configuration files.

================

That's it as far as software configuration goes.
The code will figure out everything else at compile time.

The rest will be wiring up the panels.

Be careful about the current needs. Some backlights draw quite a bit of current and
if you are powering from a USB port,
You may have to limit the the backlight current a bit more than normal so
you don't draw too much power from the USB port.

You will then be able to use the text areas to limit text output to specific regions
of an individual display.
Or let the text span them both and wrap/scroll them both.

There are some special big number fonts provided if you want to display
numbers. They are really LARGE and are good for things like a clock or other
numeric type displays.

It will be a fun project.

--- bill