I checked my copy of the book and I agree with you; the cockpit locations are not specified. It could be an oversight, but there are also reasons not to specify a standard cockpit location on a double.

In a single most paddlers will place their center of gravity in about the same location. (Usually pretty close to their belly button.) So a standard cockpit location works because the paddler load winds up close to the same location. In a double the center of gravity of a pair of paddlers can be all over the place. So there is no one answer on cockpit location.