My mobo has heatsinks on both slots, so it's a really good question... or it would be, if SSDs themselves were not equipped with their own heatsinks, that are typically impossible to remove (at least without losing warranty). This makes mobo heatsinks pretty useless, unless I was willing to buy an SSD without a heatsink, but I think such models will become more and more rare very soon. Why? Because it “looks cool”. These days even most DDR4 units come with stupid plastic “heatsinks” that are absolutely useless, and would still be even if they were real heatsinks, so what can stop SSD vendors from equipping their products with real heatsinks, where they are actually useful? Nothing, I guess.