To have multiple instances accessing one oracle database in your machine, you will need Real Application Clusters (RAC) i.e. the Oracle clusterware 10g services installed. You'll probably need to specify lots of things including what constitutes your voting disks, public network and private network for your cluster interconnect and not to mention the oracle cluster registry (OCR) disks as well.
Of course, all this is intended to allow the separate instances to run on two or more separate server machines with the datafiles out on shared storage - it doesn't make any sense to run multiple instances on one machine, unless they're for different databases. Even then, an instance is a heavyweight entity, lots of overhead even before the first session connects, and it's not overhead you want to take on unnecessarily. Unless applications are poorly designed (and admittedly many are), they're usually able to run in separate sets of schemas within the same database, so it might be worth considering that option.