For games that require multiple disks or ROMs to run, a media group can be defined.
A media group is a zip file that contains the ROMs that make up the game with an M3U file that contains a list of the files to be used by the emulator to populate the disk selection menu.
How to create a media group
On the Game info page, go to the ROMs section.

Click on Edit

Select the ROMs that will make up the media group - the ROMs must all be for the same platform

Click the Create Media Group button. The server will begin building the media group.

Click the Launch button next to the media group to start the emulator with the newly created media group.