For those looking for the two-halves style, there's a new one I just came across. I don't know anything about it other than what I read in the description, so no endorsement here. Maybe the pics will help you DIY seamsters (I'm jealous of your skills!).
COUREUR Canoe Pack - Single The Coureur Canoe Pack is also inspired by those who prefer the traditional portage packs and wanigan in the backcountry. The Coureur Canoe Pack is a medium volume pack (52L) that is a rectangular box shaped pack horizontally oriented and shaped to sit low in the...
www.rutabagashop.com
I have to say, I may be going back to my backpacking pack to solve the multiple pockets problem. The main compartment can divided into 2, with a part sized for a sleeping bag at bottom and the top of the main being tall enough for my tent. The 'brain' top has interior and exterior pockets, and lots of outside attachments including a web of small diameter bungees. The downside is that with everything packed in lightweight dry/stuff sacks it can be a little hard to fit it all.
But when I bring my Granite Gear portage pack, although it's much easier fit everything, I spend a lot of time rummaging and invariably as I pull out one thing, the smaller stuff falls to the bottom or things shift, and now I'm pulling out everything to get the one small thing. Plus if things shift something often starts jabbing me in the back, even with careful structuring when first packed. The nail in the coffin is that the portage pack doesn't lay down in the ends of my Phoenix (the dog goes immediately in front of me where a canoe pack would fit best).
Regardless of main pack, I double portage and usually bring either a thwart bag or something like a small camelback or cheap lightweight drawstring backpack to keep my 'valuables' (keys, wallet, phone, ditch kit, map, compass, snack, water, extra layer) with me at all times. Obviously the thwart bag is best while in the boat, the mini backpack nicer on the carry. It goes on the front while the main pack is on the back.
I'll note that I'm thus far a short duration tripper. I could certainly see the advantage to those split packs on a longer trip with more gear/food and more need to balance ballast.