does anyone else know of 2 nine hole courses close together that would have a different rating if combined, or an 18 hole course that possibly could/should be 2 separate nines (especially if one of the nines would get a better rating if it was divided)?
I don't know of any pairs of 9-hole courses that could be played/thought of as a single 18.
But I've played a good number of courses that had separate front & back 9s, looping back to the parking lot, so they
could be thought of in terms of separate 9-holers (though I'm not sure that they
should). A few of which have quite different front & back 9s.
In addition, I know of some courses that can offer a convenient 9-hole loop -- by selecting to play certain holes that create that navigation. (In fact, on our private course there are 3 such loops we sometimes play, when we only want to play 9 holes).
All of these offer some thought experiments of what a 9-hole course
could be; you can imagine losing the rest of the course, and think about how the remaining 9 would play. Because these courses tend to have holes designed for advanced players, whereas most 9 holers tend more towards beginners.
That also points out the limitations of 9-holers. It's not just that it's not enough disc golf, or more redundant if you play a lot. It's that a course can offer more variety with 18 holes, than is possible with just 9.