I have a workstation with two Opteron processors with RedHat AS 3.0.
When the ModelSim 64bit version is running on it, I can only see 70%~90% usage by a "vsimk" process just on one processor, while another one is almost idle.
ModelSIM is a single kernel application. Thus, it only uses 1 processor. But, if you have both licenses for ModelSIM PE and SE, you should be able to run both ModelSIM application (one must be SE, and the other MUST be PE).