Unfortunately, probably not. The ShowDown is around 75cm in length. You would need bars that are at least 165cm long to fit two ShowDown boat carriers. This may be possible on some cars using Yakima StreamLine bars, bearing in mind that roof bars shouldn't protrude further than the widest point of your car (excluding the wing mirrors).
You will also need to check that you have at least 9cm of roof bar protruding beyond the roof bar feet on each side of the bar. It's also important that you don't exceed the maximum weight limit for your car roof and roof bars.