I believe its not possible with the sparkfun shields. But i have done exactly what you are asking with the Rogue MP3 shield - http://www.roguerobotics.com/products/electronics/rmp3
they are a little bit pricey but VERY good quality. I was more than happy with my setup.
Just stack two shields on top of one arduino. You'll need to name the tracks on each SD exactly
the same and then edit your code to include that track name, but it works like a charm...
Here is my code -
SoftwareSerial rmp3_serial(6, 7);
// Serial.begin(9600); //
I seem to remember that the Rogue MP3 library has not been updated for 1.0 but i have an edited, working version, just PM me your email and i'll send it.