Hi, in this configuration you can use I2C to send commands or whole frames from the Arduino to both Rainbowduinos. Look here for more info and links: code.google.com/p/neorainbowduino/
Yes, for example you can use mtxcontrol to store 10 frames in the EEPROM on the Arduino and send those to the Rainbowduinos over I2C. Because the EEPROM is quite small, if you want more frames you need to start using PROGMEM to store the data in program memory, or an SD card shield and library.
I use my rainbowconverter program to convert animated GIFs to Rainbowduino frame buffer format and store them in PROGMEM (by generating code), then send them over I2C to the Rainwbowduinos running neorainbowduino. Look at the Daft Punk helmet project in the Rainbowduino Carnival, as I recall he went into a lot of detail about different animation formats and storage size.