Basically you install this plug-in and you can monitor both cores of your Core 2 Duo CPU temperatures. You need to have the latest RivaTuner installed prior to doing this:
Just open the file you just downloaded with Explorer, open it and RT will install it automatically then do the following:
Go to Monitoring -> Setup -> Plugins and tick C2DTemp.dll plugin. CPU temperatures (for both cores) will appear on RT's graphs after that.
Should you not be able to figure that our then the Author (Igor Levicki) has written a full step-by-step guide in this thread.