Would an ~20' bus length be doable for i2c? If not, would there be an easy way to extend the comm link so it reaches? (The device away from the Arduino can only be controlled through i2c.)
What is that little thingy '
I don't know how to calculate that into meters.
I am assuming you mean 20 feet ?
Yes, that can be done but more details are needed 
I do mean 20 feet / 6 meters. Sorry for the American abbreviation.
My plan is to have an arduino, connected to this display (http://www.adafruit.com/products/1269) with an ~12 ft/4 m cable, connected to another display with a ~6 ft/2 m cable.
That's too far for an I2C bus by itself. Look at bus drivers to add extra range.
As Mike already said 20' is a bit too far to work reliably, even at the lowest bus frequency of 100KHz.
I've used the PCA9600 I2C bus buffer chip in projects and have layout and schematics of a little board that uses this chip. However, it uses the smaller tssop 8 version of the chip so you've got to have some soldering skills.
You would need one each device on the bus , so three in your case.