- Multiple Windows Login using same schema -- Not possible.
- Multiple Database Authenticated Login using same schema -- Not possible
If you want many users to use the same schema, then use 1 common Database Authenticated Login to connect and create\load data.
Example: 5 users use a login named GISOwner, which has schema 'GISOwner', and use that to connect and create\load data. This way whatever data these 5 users create\load, everything will be under 'GISOwner'.