THe default SD library is built on top of an old version of the Sdfat library. The SD library limits the number of open files to 1 as far as I can tell. The latest Sdfat library (rather than the old one distributed with sd library) only limits the number of open files basaed on the RAM available. see http://arduino.cc/forum/index.php/topic,51087.0.html for example.
fatlib16 is the author of sdfat. sdfatlib20120719 is the latest version as far as I can tell.