i2c, 2 arduinos, oled display and pin extenders

Hi all! Im currently working on a project involving i2c and mcp230xx pin extenders, controlled from a arduino nano. Its nothing seriously advanced, its a light controller for a vehicle, turn indicators, high/low beam, brake lights and so on. I already ordered pcb´s for the project, and im almost finished with the hardware part, so i cant really change much there.

But recently ive started to desire a small display for my project, even tho its a bit late in the build stage to just throw in yet another complication... The things to display would be vehicle-stuff like speed and distance, and serve as system feedback, for example a blinking arrow when turn indicator is engaged or a triangle icon when hazard lights are on.

The display sketches uses a LOT of resources, witch leads me to consider a dual arduino solution, one arduino to be used as my light controller, and the other one as the display controller.

but the display that i ordered (but have not yet recieved) (128x64 white OLED) uses i2c as interface, witch would mean i would have two arduino´s, trying to write to the i2c bus to two different sets of slaves, if i were to use i2c as interface between the two arduino´s.

So my big question is; could i have one arduino as i2c master, the other as slave and still have both arduinos communicate with each other and their respective i2c slaves or will there be interference? As i understand things, it would mean a lot of data between the graphics unit and the display, witch i guess might block the bike light controller unit from communicating with the pin extender IC´s.

Would anyone suggest a better way for the two arduinos to communicate, if i would have to separate the two i2c buses?

I would rather not install a second cable for the display part of the system (after all the hazzle of getting my cable cluster down to a 4 core cable), but i will if i must..