Due to the way Oracle shared library referencing works, you might be able to register multiple DLLs in the same instance, BUT YOU SHOULD NOT EVER DO THIS! Oracle instances can only be registered with exactly one geodatabase -- There really ought to be only one master SDE geodatabase, but if you do have user-schema geodatabases, THEY MUST BE UPGRADED IMMEDIATELY AFTER THE MASTER GEODATABASE!
If you simply have multiple Oracle instances, then each one can have it's own version of ArcGIS, with its own referenced DLLs (the security issues of the LISTENER have removed registration there). The trick here is that there's no documentation on the procedure because it's not formally supported. I regularly ran 6-8 ArcSDE instances of different releases and databases on a single machine, but it's somewhat temperamental (especially at start-up on Windows platforms; cakewalk on Unix), and requires an arcane level of understanding of ArcSDE function.
All-in-all, your safest option is to run your geodatabases in parallel release states, and reserve the odd-man-out instance to a test server before upgrade.
- V