No, it is not possible to use same SQL Server with named instances for Domain and Enterprise Manager.
When you are using remote SQL Server MDBs, please note that the name of the server hosting the domain MDB is used as the name of the domain manager.
Therefore, if the Enterprise Server and the Domain Manager use the same database, both the domain manager and enterprise will have the same name.
In addition, CCS can run against one instance of a Database. So, if you have the Domain Manager and the Enterprise Server pointing to the same Database server, only one of them can use CCS.
Therefore, CA strongly recommends using separate MDBs for Enterprise Server and Domain Manager.
Release: UASIT.99000-11.2-Asset Intelligence