I don't know for sure about 2 WotLK Alchemy trinkets, but at the moment I'm wearing my old SSO Alchemy trinket,
Redeemer's Alchemist Stone together with my new WotLK Alchemy trinket,
Mercurial Alchemist Stone. The 40% add on potions does not stack as far as I've noticed, but the rest does. So I'd assume it's possible to wear 2 different WotLK Alchemy trinkets as well.