Increasing the capacity of the axles and tires by doubling them up doesn't double the carrying capacity of the entire trailer. As they say, a chain is only as strong as the weakest link. What do you need to accomplish?
I wouldn't worry too much about it. If your boat is properly sized for a single axle trailer and strapped down properly, neither the boat nor trailer are going to go anywhere when a tire blows. And besides, when tire replacement time comes, would you rather buy 2 new tires or 4?
