The Kickstarter says you can't pit two planes of the same side against each other... Is there something in the maths that stops that (I can't see why there would be, from the main post) or is it purely because the book doesn't have the right art in it?

it's because of the maths: I do feel like the article overcomplicated things a bit: the basic idea is, each individual book is basically a choose-your-own-adventure of flying around a static opposing aircraft. If you pick page numbers yourself you can happily just fly around your 'opponent', who never moves on their own. But, the page numbers for opposing books correspond to the inverse of the move in the other one: so when you take the page number from your opponent, you are doing the inverse of their move on your book (which may not be an option listed on your page). This basically means there are two opposite maps: if you tried to play with two books from the same side it would seem like you are just wrestling over the controls of one plane.

That's right, but you can pit a rotary axis against a powerhouse allies. You can also have group battles.

That's great. Back in 2014 they kickstarted a Handy Rotary with a different production crew, but the screwed up the page order at the last minute so that the page numbers are on the inside edges of the page. I'm happy to see these started up again.

I grabbed the same version. I didn’t notice the page number thing, I’ll check it out next time I am in storage.