How are you managing your songs; are they both in the same pack or different packs? You need to manually change the path in the DTA (sZGE/000/) to the right number for the pack you’re making; though you have to remember: the app filename is hexadecimal and the path in the DTA file in decimal, they must match. Once you changed one song in a single pack; you can use C3 CON tools to merge the songs together; and using app files you can merge a good number of them.
You are about to leave Rhythm Gaming World and access the external songs DB. While no song or file sold for commercially available videogames is hosted there and only fan generated content is allowed, Rhythm Gaming World has no direct control over the database and no files are hosted here. Alright, we had to made you aware of this, now you can visit the database!