Just got the BRD +block trinket, and wouldn't you know? They stack. Just a heads up, so you've got something to grind for, specifically for Wotlk where +block value matters even more than it does now.
They do share cooldown, but they still stack. Tried with pvp gear on and both trinkets, 280 block value without, 680 with both activated. If it's not a bug, rejoice. If it is, rejoice as long as it lasts, and hope Blizzard wont notice for a while :p