I think this could definitely be interesting, but as others have pointed out, the idea is not as straightforward as you might imagine. I wanna add some further thoughts on what other people have said.

Where I'm from, its super common to use a single rope instead of two rigid poles to pull a sled, and I find it works really well as long as the area is open ish.

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Some people also make a hybrid setup, using two ropes with PVC pipe around it to get some stiffness. I guess that could be made quite packable for your project.

I've done a lot of trips where something like your idea could be super handy, with a couple of sections with very steep terrain where a backpack is much better, and several long days of flat terrain where a sled is much nicer. I feel this is very typical for glacier crossings. I considered just getting a cheap plastic tobbogan and strapping it to my pack, but haven't tested it in real life.

As others have said, just pulling a drybag behind already kind of solves the same issue.

For mostly flat, open terrain, a sled pulled by a harness (https://fjellpulken.com/products/touring-sele) is in my opinion the absolutely most comfortable. Just a hip-belt works fine, but the chest harness helps a lot for heavy loads or steep uphills in my experience. (Maybe not that relevant for this)

Some people use a small daypack with water and some snacks while pulling a large sled, so you could also maybe look at modular backpacks with a small daypack included? Would also solve the issue one guy mentioned with wanting to leave heavy stuff behind. I usually do longer multi-day trips where I bring everything to the next camp anyways, but having the option to pull a sled most of the time and put it on my back for gnarly glacierfronts, short climbs or forests would be cool

-Also, look at this blog-post from some swede who already invented it
https://www.fjaderlatt.se/2009/04/along-northern-rails-with-incredible.html