The closer the cockpits, the better the double kayaks handles going up and down the waves, but the paddlers will hit paddles if not in synch, and in the right wind, close cockpits will cause one of the paddlers to get wet from water flying off the other paddler's paddle. I use 12" back from CB for the backband, but your 14" is fine. Like you said, center the distance between cockpits on the CB based on the CB for each paddler. A rudder on a double is a good idea. I planned to install one, bought one, but couldn't bare to put it on my double, even after having a hard time broaching from quartering seas crossing back 19 NM from Catalina Island. With more relative weight in the kayak, that weight farther at the ends, and the faster speed, doubles just don't behave as well as singles in conditions. Even Nigel Dennis Kayaks writes about their double, "A rudder might also be preferable if you intend using the kayak in greater than force 5 [17-21 knot] winds."