Ok, I ran 3dmark06 and as I suspected I get identical scores before and after 'overclocking'. (8550 pts)
This is what the overclocking tab looks like in Rivatuner:
I've only altered the 3d performance settings, but looking at the hardware monitor after a run of 3dmark06:
You can see that during a run of 3dmark06 the core/memory Mhz don't go up and stay at the stock 500/600 setting...
Also, when I run GPU-Z I can select both GPU's via a dropdown menu (like with a dualcore on CPU-Z) Only the 2nd from the dropdown menu shows the new frequencies for the core and memory. So at first I thought that perhaps I only overclocked one core but with the 3dmark06 scores being exactly the same that doesn't seem to be the case...