http://www.wowhead.com/?item=40901
Using shred on a ripped target adds the 2 seconds, as intended. However, it is not capped at 6 seconds added. 4 Shreds will add 8 seconds to Rip, 5 adds 10, etc.
Not a big deal, since most ferals don't have the energy to keep stacking shred every 2 seconds, but still a bug.