Mine is a Garcia and I'm sure it is not waterproof. I keep my food in my pack except at night - those things suck to put in your pack. My food is always sealed in plastic bags so there is no need (in my mind) bag it again and my pack will float. I'm using my backpacking pack for canoe tripping. I used to have a 90L bag, now I'm using a 65L bag. My wife has the same size. I can carry my canoe and pack for quite a ways (about 70lbs total)... two miles might need a couple breaks.
The straps would necessitate a double carry for the barrel because my wife has her pack to carry.
I don't know if this would help in your case, but I have a Garcia with the nylon carrying case. I clip it to the sternum strap on my Granite Gear Portage Pack. I carry a week's worth of dry food plus toiletries plus a trash bag in it, and manage it on portages up to 2.5 miles at a time. Visibility is not a problem. Also carry a full 750ml fanny pack by a shoulder strap - that is a nuisance, but I deal with it. My 12 lb Hornbeck pond hopper does not appreciably factor in to the weight total.
If each of you had a bear cannister clipped on the front, one of you might be able to single-carry, but I would hate to think of the other trying to single carry with all I described and a real tandem canoe.
YC, I think I recall something about how you and your husband sometimes arrange things so that you go ahead faster on a portage while he starts out slower with the tandem canoe plus gear, and then you double back empty handed to meet him halfway and lighten his load of the extra gear, so the two of you can complete the portage together. Did I get that right?
I don't know if that sort of strategy might help your situation, l'oiseau. If having two bear cannisters doesn't sound appealing, maybe having one barrel but having the person who doesn't carry the canoe carry the barrel and use the strategy described above?
As our old scoutmaster used to say, it was a idea, maybe not a good one, but an idea.