At the same time? It won't hurt anything, but it actually might not work as well as both alone.
Here's a guide for linXhttp://www.overclock.net/intel-cpus/645392-how-run-linpack-stress-test-linx.html
For reference: at 3.6 GHz, my i5 gets around 51-52.5 GFlops. If you don't get very near that at 3.6 GHz, the test is not running properly. If you can pass 50 tests at 51-52 GFlops, your chip is quite stable.