Assuming Reconciliation Rules (Reconciliation Definitions) were configured exactly the same for both Data Sources, ServiceNow...

If an Application CI is discovered with a Data Source, ServiceNow, then if the same Application CI is discovered by the Data Source, Altiris, an update will be allowed.

If an Application CI is discovered with a Data Source, ServiceWatch, then if the same Application CI is discovered by the Data Source, ServiceNow, an update will be allowed.

If an Application CI is discovered with a Data Source, ServiceNow, then if the same Application CI is discovered by the Data Source, ServiceWatch, an update will be allowed.

If an Application CI is discovered with a Data Source, ServiceWatch, then if the same Application CI is discovered by the Data Source, Altiris, an update will be allowed.