So if I understand you, you're doing:
You've got 3 RB devices, but 4 cards, and two completely separate wireless links. Your first post made it sound like the middle RB(2) had just a single antenna and was talking to two stations with it, instead of two separate point-to-point antennas/links.
Why not try running MPLS on all three RB devices with a VPLS bridge between the two end-point devices, RB(1) and RB(3)? Then the middle one will only need to switch MPLS packets while the two end RBs will terminate the VPLS bridge.
Since each wireless link is point-to-point, you can pick and choose which side is the AP and which is the station separately for each. The middle RB(2) could be acting as AP for one link, and STATION for the other, or STATION on both, or AP on both.