Seems like an invitation for trouble to allow two Arduinos to become 'masters' on the SPI bus. I would suggest using a master-slave configuration as intended by the folk who brought you SPI. Another option to consider is communicating between the Arduinos using I2C and EasyTransfer by Bill Porter. Easy, straight-forward, and very reliable in my experience.
Have you tried using a 'larger' Mega for your project? Might be the simplest solution of all. Or, if you really want to get fancy, the Teensy 3.0, Maple, or Digilent lines of very powerful processors programmable with a Arduino-like IDE?