Within the same Oracle instance and within the same Oracle database, you can have multiple user-schema geodatabases but one and only one SDE schema (referred to often as the master schema). In other words, you have one master geodatabase and multiple user-schema geodatabases which are essentially system-managed through the SDE schema. All of those user schemas MUST be at the same Esri release; you cannot have one user schema at 9.3.1, one at 10.0, and another at 10.1 SP1 while the SDE user schema is at 10.2. Upgrading the geodatabase will migrate all user schemas to the same version (at 10.1 and higher, the Upgrade Geodatabase tool does this automatically) and it's not possible to use multiple releases, period.