FIM Synchronization Service cannot be installed on the same database instance which hosts FIMSynchronizationService database for another instance of FIM Synchronization Service.
This article provides guidance on how to install FIM Sync Service with an existing database.
In-order to have two instance of FIM Synchronization Service instance running, the database needs to be installed on two difference SQL instances.
Microsoft doesn’t recommend have two instance for FIM Sync Service. However, it might be helpful to have two instance for migration of the production instance from code based to code-less provisioning.
Case in point, the production server with provisioning logic built based on the old FIM/MIIS methodology. To keep the production sync running in an uninterrupted manner during this migration, you could install FIM Sync Service, FIM Service & FIM Portal on a different box.
One major difference between the two sync services is only the production will be performing the Exports. This will give enough time for the code-less logic to buildup all the metaverse as all the importing of objects and subsequent application of the new FIM policies might take quite sometime.
Once the metaverse is all built up, the database could be easily migrated to the production FIM Sync server.
