There are some games in Japan (particularly pirated games), that have more than one game on one disc.
I don't think I've ever seen multiple PS2 games on one disc but I damn sure remember seeing multiple PS1 games on a single CD.
I tried one of these discs on my ancient PSX console and it did work. What it basically does is once the disc boots up, it comes to a menu where you can choose your game out of the selection. Once you've made your selection, the game starts as it would normally. But if you want to play another game from that same disc, then you need to reset the console.
Make no mistake about it, it's very possible. But unfortunately for the original poster, I have no idea how it was done.