i dont OC much, but if your gpus are in crossfire , you are using both of those gpus.
while OCing , if your system shuts down , you mostly have high tems and need a better cooler or you need to reduce the oc
if you get artifacts, step the OC down a bit. NOTE- sometimes if it is too high it might cause permanent damage
more than insane stress tests like furmark , i suggest running demanding games like ac unity or dragon age