@mineown Welcome to fluther. It’s hard to ask questions, but give it time – read others’ questions to get a feel for it, and soon you’ll be in the 10k club and fluthering with the best of ‘em. Hang in there.
I’ve always wondered that myself, and Wild Arms wasn’t the only one disc game to come in a double case…it may be because the Japanese version came with some soundtrack, bonus anime disc or the making of or something, but the American audience was denied the extras, yet they were too half assed to produce single cases.