They are tested to work stable in dual mode without causing errors and are then sold as a package. They also come with a heatspreader (cooling), which makes them better for overclocking. This doesn't mean of course that 2 Corsair 512Mb DDR400 Value Select won't work stable in dual mode, it's just not guaranteed. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
