Typically, You would first rip the game to your xbox HD and then FTP all of the files from the xbox to a PC for working on.
Then, depending on how the multi-game disc was made, you will first see a default.xbe in the root of the directory. This file is first to be executed which in turn, launches the game chosen to play located in a sub-folder.
Then you should also see folders labeled game1, game2, game3, etc(or something to this effect). These folders will contain the complete individual games. So, you would simply create your new ISO's from each of these folders.
That is how I have done it in the past but like I said, it depends on what software was used to compile the multi-game structure in the original ISO.
Also, keep in mind that these games, once extracted from the multi-game disc, will be quite small and frankly, a waste of a dvd, hence, the whole "multi game disc" thing.