Yes the Rezound has two SD cards, one 16GB internal and one 16GB removeable. The internal card is addressed by either /sdcard or /mnt/sdcard, and the removeable card is addressed by /sdcard2 and sometimes by /mnt/sdext. I say sometimes because originally I could address the removeable sd with /mnt/sdext, but the address seems to be no longer available.
Since most apps that told to use the sdcard will default to /mnt/sdcard, they will end up on the internal sdcard. However most apps allow you to pick a custom folder, so just pick /sdcard2 if you want the removeable sdcard.
For media like music, photos, etc the apps generally search all phone storage and will find your media regardless of which sdcard it happens to be on.
You can change the removeable sdcard for a larger and/or faster one if you like. I use a 32GB class10 card for the removeable storage giveing me 48GB total storage on the phone. Note that approx 6GB from the internal sdcard is used by the phone leaving around 10GB available.