Peter, the amount of volume you are practically trying to displace with flotation is pretty huge. Seems to me that portioning off the bow and stern with bulkheads is a necessity and first place to start. Ensuring the bow bulkhead is as close as possible for the largest paddler. After that intall interior rigging tiedowns or bungies so that waterproof containers and drybags can be central and held in place in case the boat is tumbled. There's a lot of usable space all around the interior of a double. Having misc. items floating around isn't helpful. Consider water removal methods other than hand bilge pumps, electric or foot.

The second would be to review your rescue skills in doubles so any solution is based on experience. Everything else is assumption.

When I hear folks thinking of expanding foam it conjure us images of trying to make a sit on top with a closed deck boat,,somethings not right.