I never used anything like that, but it seems it would have some advantages, like resisting racking better than the single-spine method. It might complicate setting up the forms, but maybe it wouldn't be much different. Definitely worth a try!
I couldn't find 2x anythings that were straight enough for a strongback, and ended up gluing long pieces of 3/4" plywood together to get the thickness required. It was heavy, but it was dead straight. That structural strength may have helped get (and keep) the forms aligned, which is really critical.
I've not used that style strongback for canoes but use a similar system for building double ender row boats. It's a one of a number of strongback styles used in building traditional all wood boats. Works great, it's easy to set-up, easy to move the partly built boat should the need arise, you don't have square edges in the way for those tight ends... I think it would be excellent for canoes.
That's an interesting design, I've never seen on like that before, (not that that means much).
I started with the commonly seen T cross section, and decided it wasn't stiff enough, and so added some bracing, but that didn't add any torsional stiffness, so I boxed it in. If I were doing it agin, I would just make the plywood box version. Mine is very heavy and is not readily movable. And in fact, when I build on it, I glue it to the floor so it doesn't move. (My floor is a "bit" uneven so any change in location of the feet of the beam would put it out of level.)
It doesn't really matter if the strongback is level at all as long as it is stable. What really matters is that the forms are level and in the correct position. For repeat builds, then a stable and level strongback is a a real boon as you will use it as a reference. For a one off, you trade one kind of work for another.