I'm *trying* to do it one then the other - I'm wrapping the individual parts in function calls, and calling the ones for I2C THEN the ones for SPI.
I'm using different pins for each bus.
I'm using hardware i2c pins (analog 4 + 5 on the duemilanove - as used by the wire.h lib).
It's great that it's possible! It means I'm not barking up the wrong tree.
I'm just having an overhaul/debug of my code, and then I'll post it/the relevant bits. It's not pretty, but I'm afraid this is my first bit arduino project.