There is no easy way to achieve this. I can not talk for Oracle (however I would assume the same), but SQL gets installed as a service and is available for everyone on the machine.
If users are administrators, there is no real way to prevent them from accessing each one, even then, unless the database is secured (encrypted, usernames, passwords etc.) it is possible they can just take a backup of all data.
If they are just users, I would recommend that you install as normal under an administration account, then you could put both services to manual startup and run a logon script for each user that just has "net start sqlexpress" etc, or along these lines.
You may be able to go further by starting the services under the account name of that user and using NTFS permission on the program folders so each user cannot access the other database and will not have permission to start the database.
I know SQL is compatible with this - I do something similar on a domain, however I have never run it as a limited user. Again, I can not talk or give advice with Oracle, but I would assume it was similar.