Two definitions first, because the word co-host does double duty. On Airbnb, a co-host is a specific permission level you can be granted on someone else's listing, letting you message guests, edit the calendar, and in some cases receive a share of the payout. In the wider market, co-hosting means you run the whole operation for a fee, usually 10 to 25 percent of revenue, and the platform permission is just the plumbing that lets you do it. Your 15% offer is the second thing.
Startup cost is lower than most service businesses because the owner's money buys the furniture and the cleaner is paid out of the guest cleaning fee. What comes out of your pocket is software, business insurance, and your own time. Expect a dynamic pricing tool and some form of guest messaging automation, an entity and general liability policy, and a laptop you already own. That's a few hundred dollars a month at two units, not a few thousand.
The scale question is really about fixed cost per unit and how thin your attention gets. Two properties means you personally answer the 1am message about the hot tub, because there's no revenue to pay anyone else to do it. Somewhere around eight to twelve units, most people can afford a part time person for overnight and weekend coverage, and that's the point it stops being a job you can't leave.
One thing to settle before you say yes: whether these cabins are legal short-term rentals under the local ordinance, and who holds the permit. Registration rules differ by city and county, and you don't want your first client to be an operation that gets shut off.