Normally you need to merge the directory structures of both CD's to a single directory structure on your hard disk.
Now since everything is on one disk already it means it is already merged.....but probably in a way that ScummVM does not understand.
So you need to take a look
here and figure out what files you need to move and/or rename.
BTW, what are you missing in your version? they must have removed the movies and/or speech to fit it on one CD.
Or is it a DVD by any chance?