I agree with Demon, try to look at the whole as a bunch of functions and locate the function on the CPU best placed to deal with a particular piece of hardware, for instance move the LCD handler and fonts etc. to a supernumerary cpu attached to the LCD's. As you are going to have 2 ? CPU's suggest SPI (a good deal quicker than I2C) slave /master. You'll need to establish some protocol, like first byte is command (in this case data to LCD), second byte is length of data, data n bytes (inevitably this will get more complex). You'll also need a mechanism to allow data to go the other way.

Actually the more I think about this, the more positive this could be. While leveraging your existing programming skills, you'll inevitably you'll get a performance hike, with all the humdrum stuff removed (to the other CPU) debugging becomes easier, if the second CPU is a separate board it makes it easier to add functions and a whole bunch of other stuff I haven't thought off.

I know the old corporate nonsense about challenges and opportunities but I think in this case viewing the situation positively could result in all sorts of benefits.

I wish you the best in your project, it sound really interesting. If possible post your progress, I for one am keen to hear where this story goes!

George