Use different channels, and you can think of each link independantly.
You could have your RB and NRBs on either side of the link, but with
your present description (Roots at the ends), both RBs can run their
channel optimization algorithm and select the best bi-directional
throughput channels for both connections. If both center bridges
are using the same pole, it's recommended to try and separate them
by 2m (6 feet) if you can.