[GreyingJay]

Connect them both to the Ethernet. (If you run out of ports on the robot radio, install a small Ethernet switch.) Set the Pi to use a static IP such as 10.te.am.200 (something well out of the way of the FRC DHCP assigned IPs).

You could in theory use I2C, DIOs, PWMs, USB etc. but Ethernet is by far the simplest.