I have two users in a Oracle DB user_1 and user_2. This issue occurs when I create a two tables by user_1 its show one table as user_1.table_1 and the second table as user_2.table_2. I'm not sure why this is occurring?
When you do not specify a target schema in the datastore configuration GeoServer picks all tables, but does not qualify them by schema, not even internally. So it will pick the first table it finds.
If you are interested in having schema qualified tables support see: https://github.com/geoserver/geoserver/wiki/Successfully-requesting-and-integrating-new-features-and-improvements-in-GeoServer