If I upgrade to Cubase 9 from 8.5, will I still have access to 8.5? Will I have 2 separate icons or only Cubase 9?
Yes and yes. 
Yes you will have access and yes… (2) icons.
Regards 
Yes, I’m running both simultaneously now actually (on Mac). I also still go back and use 7.5 (which I really love for editing) when I get annoyed with the cursor/timeline disconnect issue and GUI lag that’s plagued CB since 8.0.
Note, after installing and using 9, when going back to 8.5 CB it had to re-scan all my plugs again. Took some time, but all opened fine.