In situations like racing classes, the beam of open canoes is usually measured at the waterline (usually 3" or 4" up from the bottom). A tandem pro marathon canoe, for example, is sometimes called a 3-27 boat, as it's beam has to be a minimum of 27" at its 3" waterline. For most recreational tandem canoes it is probably more common to place their maximum beam at about the 4" waterline, and the official catalog beam would be whatever that maximum was. A 4" waterline beam of 32" or better is about where tandem stability starts. Narrower is possible, but with two people for recreational use, its stability is likely to be a bit twitchy.
Weights listed by every canoe company that I ever was a dealer for (Old Town, Lincoln, Sawyer, We-No-Nah, Wilderness Boats, Hyperform, Phoenix, Klepper Grumman, Beaver, etc) were the all-up weights with everything attached that was not intended to be removable - as it would be if you had to portage it. Just how close the actual boat weights were to the published catalog weights varied a little bit, but were usually pretty close. Weighing just the hull would be pretty pointless and I have never heard of anyone doing it, unless maybe they had various levels of trim-out that you could choose from and add to arrive at the actual weight.