I tried taking my G4 OS 9 and tried to install it on my G3 and it was severely messed up. Couldn't connect to the Internet because it couldn't find the internal modem, among many other problems.
Then I just tried to reinstall 8.5 and then 8.6, but they refuse to overwrite some files, and you'll have to do a fresh intall zeroing out your HD.
I then went to purchase OS 9.0.4 CD (which I planned to do anyway) and that works great.
Plus it's illegal to put the OS software on 2 machines that way.