As it says on the specs
here:
I2C: 2 channels (1 dedicated, 1 shared)
<many other similar examples>
So 1 channel is available on the roboRIO, and is a dedicated I2C bus. There is also another on the MXP, which is a separate bus, but it's pins may be used as Digital I/O instead, if you want to. Not D I/O and I2C at the same time. But, there are two I2C channels.
The MXP is an expansion port, not just a move-all-the-onboard-IO-somewhere-else port.
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