The game is natively multi-thread. No need to try and coerce it to run on more than one CPU. It does it already. However, there is a bug in all dual-core AMD CPU's we happen to hit. There are two patches from Microsoft and a patch from AMD, which when all installed will work around the bug, for the most part. You will never get the performance from a dual-core AMD CPU that you would from an Intel dual-core, due to the bug.
Dual-core Intel CPU's do not have any issues. Quad core AMD and Intel CPU's do not have any issues either.