HostelSystem was built for people who may run more then one hostel. Therefore, not only you can run multiple instances of HostelSystem at one computer and see both hostels at the same time.
In order to log in to selected hostel simply select the hostel from the list after the startup:
If after opening HostelSystem, you see the user selection right away, then click on the link in the top-right corner:
If you are logged in the system, then select Switch Hostel option from under the logo in the top-left corner: