First - Do you agree with this?
Yes, I do. Observing myself, I am a clear interplay between my wings. And at times, I feel both my wings happening more than my core type.
Second - If you do, has it ever seemed strange to you that 8 and 9 are next to each other? They seem so different to me. 8 seems to be the most aggressive of the types and 9 seems to be the most diplomatic or "nice". When I think of a 9w8 or 8w9, it seems like an odd pairing.
If you go by outward behaviors of conflict versus peace, then yes it is strange.
You have to go below the surface here. It's about resistance to the environment, inertia, denial of circumstances, and numbing out or forgetting whatever doesn't agree (for 9s, this is whatever ruins their inner sense of peace; for 8s, this is information that might set them against themselves).
The 8 is caught between the high-energy, enthusiasm, and big ideas of the 7, as well as the inertia and routine of the 9. Consequently, you get someone who has lots of grand ideas, but is often settled enough to actually accomplish them. The "gluttony" is held in place by inertia, hence you get excess--instead of doing everything, 8 do one thing over and over again, then move on to conquer something else. Given that both 7 and 9 tend to put their minds elsewhere, you have someone who is very unable to be affected by the present moment (hence, callousness)
The 9 is caught between the "goodness" of 1, and the "badness" of 8. They don't know whether to agree or disagree, to obey or disobey, indulge physical desires or deny physical desires, etc. On one side there's the impulse to be self-interested, on the other there's the impulse to make the world more perfect. How do you deal with all that push-pull other than become numb and stubborn? Outwardly comply, but inwardly follow your own way. 9s can be both callous to others, like 8s, and concerned with maintaining the "right" way of doing things, like 1s.
It all makes sense to me.