Ok, good experiment.
I actually already use two SD cards. The second one has my audio books on it, which means that when I want to stop listening to the books, I can just swap the source to the other card and play music. That enables me to keep my place in the book. Very useful when the wife or daughter gets in the card and doesn't want to listen to 20 mins of the middle of a book they've never read
I don't think having your music split across two sources would be too much bother though would it? Just think of a way of making a logical split....Thrash Metal, Speed Metal and just Metal on SD1, Death Metal, Prog Metal, Power Metal, Black Metal and Nu Metal on SD2
(keep it metal!)