[PAR_WIG1350]

Think more like '07, or, mount the bar so your alliance partner hangs next to you. using 2 grippers, grabbing 2 different sides of the tower, will counter any balance issues.
...@................@............................. ..............
....Y=========Y==O................................ ......
........................../........................................
........................./.........................................
......................../..........................................
......................./................O=======@..........
....................../................/I.............Y...........
.....................O==========......_____I_____. ..
......................I__ robot1___I.....I__robot2___I..
......................\_/...........\_/......\_/..........\_/..
......................(_)...........(_)......(_).. .......(_)..
O hinge/joint
@ Bar
Y grabber
(_) Wheel
.... nothing
robot 1 hangs from the tower
robot 2 hangs from robot 1

Now, robot 1's grabber is redundant to some extent, if the right, load bearing gripper fails, robot 2 will hit the ground, but robot 1 won't fall on top of it. if both of robot 1's grippers fail, they still only fall near each other, rather than directly on top of each other.