It's quite simple really, if memory serves me correctly, homemade MenuX multi-game discs are laid out like this:
In the root directory of the disc, you usually have a few folders which will typically be labeled something like "game 1", "game 2", "game 3" etc, and then a default.xbe file at the bottom of the list, which is just the GUI game selection menu launcher.
Basically, the contents of each one of those folders contains a complete game but since they are generically named, you will have to figure out what games are in what folder.
So you would first rip the entire MediaX game disc to your default "games" directory on the Xbox. Once this is done, go to your PC and FTP to the Xbox and navigate to your games directory. Download to your PC's desktop, a copy of the new folder you created when you ripped your multi-game disc to your xbox HD, probably titled MediaX something or other. The folder contents shouldbe something similar the the pic below:
Now it is time to figure out the game contained inside each folder, so go back to your Xbox and using a file manager like the one in Unleash X or XBMC, start with the first folder (possibly labeled "game 1" or similar) inside your multi-game folder. Open it and launch the default.xbe file inside. When the game starts, make note of which game it is, then return to your PC, and rename the "game 1" folder to the game name you just made note of. At this point you should have something similar to the example progress depicted below.
Repeat this process for the remaining games until you have isolated them all from the multi-game disc see the pic below.
You have pretty much finished. At this point, you can simply delete the original MediaX multi-game folder on your Xbox, and then FTP the newly renamed game folders from your PC back to your Xbox in the root of your "games" directory and you're done. Those games are ready to play or you could use a program like Qwix to make Iso's of them and burn then each to their own DVD.
If you aren't familiar with FTP, this entire post can be followed solely on your Xbox using the awesome file manager in either XBMC or the Unleash X dash. You can do all of the various folder moving/renaming functions right in your file manager. As you figure out what each game is, simply move it to the root of your games directory after you rename it.