pito:
@ two sdcards to run in parallel: frankly, not sure the arduino sdfat is prepared for two sdcards connected (two cards selects is not enough, though). Imagine you have two cards, opened, one for read, the second for write. You need 2 buffers, and the sdfat driver in basically two different states (two different instances of the sdfat driver).. It might be a challenge..
My understanding was that the main idea was to be able to support either card -- one might not populate both card holders, but this gave the user the choice of which style. And I gather that given the wealth of pins, it made sense for the hardware to support both simultaneously, even if it's a "future" on the software side.