Depending on the water, it might be plausible to tow the BOB with your raft.
This idea is a little nuts, but if you in calm water and if you are using a large dry bag anyways with the BOB, you may be able to securely close the drybag and keep it attached to the Bob for floatation, and then affix the whole thing to the tail of your raft via the 2 rear tie down points. This might work to cross a lake or float a short distance on a deep slow river.