You can install one without regard to the other. I have two machines running X/9.1 and one I installed in one order, the other I did the other way. I honestly couldn't tell you which I did which way, but both work fine.
I do recall that the end result was a little different, as one thinks it's right and moves the other's stuff, where (I think) 9 just puts itself in at the root and doesn't move anything. If you install 9 after X you end up with more silly crap in /
Anyway, don't let all of the newness fool you, the old macOS is still the old macOS. A dragged system folder from another machine will work as well as it ever did. Booting from other drives / other system folders on the same drive or partition works wuite well. If you're scared, you can install a clean version of 9.1 and pcik which one to boot from.
Final analysis - it's all good.