To run the Windows SMP client, you must run a different executable, available on this page
. If I understand your post correctly, you're currently simply running 2 instances of the standard console client, each of which will (usually) run on its own core, and not interact, and which will produce less than 1/3 of the daily point totals the SMP client will on those C2D processors.
I always add the "-verbosity 9" parameter to any folding instance I run, so that any errors that occur will generate all possible messages available. It may be that if you do that the "terminated unexpectedly" error will list more information about it in the instance's FAHlog.txt file.
I've never run the Windows SMP client, and last I've seen people are frustrated with its unreliability, which is why many who run dedicated machines either run them as Linux machines for better production and greater reliability, or run Linux in a Virtual Machine hosted on Windows, which is what I do on the machine I'm posting from. Not many people who have control over machines others use will run a Linux VM, though, because it does eat cycles and often makes the machine noticeably sluggish, and the users will complain. But you'll probably want to find a better source than me for information about the Windows SMP clients available at the aforementioned link.