What difference does it make that I have two SD cards
By the sound of it, its like as if the handset is in fact a 16GB model variant, if so, then the so-called sdcard is treated as internal storage i.e. sdcard, and the external SDCard is symbolically linked to extSdCard.

That is the way Google are doing it now, and doing away with having external SDCard's... :)

No its not true that one is "more" important than the other, its just a way of increasing internal storage capacity for apps and media etc, the external SDCard acts as a "overflow" bucket once the storage space on the internal sdcard gets maxed out.