I'm not sure if you'd figured this out on your own yet, but if you haven't..hopefully this will help!
Create a Command Button without the wizard. Right-click for Properties and then set the Hyperlink Address to your other database file.
Use the ... next to the Hyperlink Subaddress property to set the "BookMark" in the mid-right edge of the Hyperlink screen. It will open a dialog box with all of the objects in the database that you specified.
Select the Form you wish to open!
NOTE: This will open the other database. You will want to provide a button on the form that not only closes the form, but Exits the application(so that the second database closes when they exit the form). So, set the minimize and close properties on the second from to NO.
Also keep in mind that if you have Access Security in use on the second database....users will have to login. If they both contain security, then make sure that they share the same users and permissions.