I believe you need to think a little deeper about your scenario. They will not both be at that speed for the entire hill - in fact probably not till near the end will the difference be very large, the person pedaling would have used up significant energy to go faster downhill (the lion's share going to defeat wind resistance) and it's unlikely that a hill that can be coasted at only 30mph can be pedaled as high as 42mph. I'd hate to be that rider who used all their energy to go a bit faster downhill having to go up the next hill. Generally it's only in a race, where one needs to keep with the pack, where pedaling downhill is advantageous.

http://www.joefrielsblog.com/2010/08...-downhill.html