Prime95 is normally the norm for testing overclock stability. However you have to set it up to run 2 instances 1 for each core. You would need to create 2 desktop shortcuts. Then label the first one Prime95-1 and the second Prime95-2. After you do that you would need to right click the shortcut go to properties under the target add this in. In Prime95-1 at the end of the target line after Prime95.EXE" put one space then type -A0 thats a zero then in Prime95-2 at the end of the target line put a space then -A1 Then you need to open each prime go to advanced affinity and target prime95-1 to run on cpu0 then on prime95-2 go advanced affinity uncheck and target at cpu1. Then you need to go to Prime95-1 go to the options and click on the torture test. After you click this you need to select blend mode. Check your available memory say you have 2gbs with 1.5gbs free you need to divide this number in half sense you will be running 2 tests one for each core. So half would be 750 megs on Prime95-1 then 750 megs on Prime95-2. I always allow a bit of leeway though so I knock of 50megs so I run 700 megs on each core leaving me 100megs of ram free so I don't have to access the page file.
This is the only way to truly use both cores at 100%
An easier option is this! I use Sandra a excellent tester. This test way more than your cpu you can test your memory bandwith and all kinds of other things. It includes a burn test which will tax your cpu 100% and you can even pre program it to shutdown if your cpu or board goes over a stated temp. Heres a download link for it. If you want to run a extended test you can specifiy how many hrs you want it to run.
Just scroll down to were you see primary download site. It will then redirect you to the download.