There should be little conflict when installing two versions of oracle on the same machine. Make sure you use different ORACLE_HOMEs for each version. When installing two versions, I typically use the listener with the HIGHER version to listen for both databases.
Creating two databases (SIDs) on the same box is also no problem. You have to watch out to make sure you have enough physical resources (memory, mount points, etc) and that your kernal parameters are configured such that you can use enough resources. Other than that, it's common practice and will allow you to maximize your oracle license.
By "maximize your oracle license" I mean hosting multiple databases on the same box. Early in the sales cycle, a lot of management types are lead to believe that you must have a seperate server for each database you are running.
Since you license Oracle by CPU (either contrived or directly), you save licensing costs by utilizing more processor per box. Of course, this doesn't make much sense if your application is already CPU bound, but on a system with idle CPU, you can add a second database to the same box without worrying about licensing costs.