I tried to install Oracle 9i on a Windows 2000 server that already had a working version of 8i running on it. However, it did not install properly and it returned some errors (sorry don't remember the errors) when I tried to start it up. I installed 9i on a completely different path (8i is in D:oracleora81. I tried installing 9i in D:ora9i.)
I read that they both have to have the same Oracle_base. In my case that would be D:oracleora81 and D:oracleora9i. Is this correct? Do I have to consider anything else for both 8i and 9i to work OK?
Oracle8i and Oracle9i do not have to share the same anything, but it would be to your advantage and would comply with OFA setup to have one ORACLE_BASE on your system. You can install 8i on drive c: and 9i on drive d: and 10g on drive m: without any issues. I have done that myself and had no problems.
I'd expect that the errors you got probably will lead you to the solution if you read them carefully and/or search Metalink for some solutions.