Ok being it looks like the only person here who does play both games let me fill you in.
WoW did NOT at launch support both cores. It does now. As of 2.3 they added dual core support and it made a noticable improvement in the game on dual core systems. Now this is not to say the entire game is multi-threading but enough of it is that all dual core people saw a performance increase.
EQ2 does NOT support dual cores. Matter of fact they went out of their way to patch it way back in LU28 to make it exclusively single core to get around a bug with the core's timing and people exploiting it to make them run faster. They could of simply required you to install the dual core optimizer or required software to run, instead they changed how the game processes. This means in a dual core system it will only run on one core at all instead of being load balanced at 50% per core like most other things. Seemingly randomly this causes issues with some people with dual cores who end up with 50% on one core and 0% the other core. My desktop has this bug with eq2 and has had it since lu28 sadly.
Unfortunatly EQ2 is the game that could really use dual core support. It's so cpu heavy even a bit of multi-threading could really help out game performance. Pretty much all new pc's are shipping with dual cores and have been for almost a year now. It's not technology that's going away and making use of it could really help turn the game around for many people.