I used to run two instances, an 8.1.6 and an 8.1.7 (mainly due to sloth, putting off moving the sites running on 8.1.6 to the 8.1.7 instance). It was pretty easy to set up. The main things were making sure that memory configs were such that both could live in memory happily without swapping at the OS level, and that the right ORACLE_HOME was set when running sqlplus/aolserver/whatever. I had everything local (the bequeath adapter), so I didn't get into any listener / network configuration issues (but I don't think that should be too much of a problem)
