Adding a second axle does not change the tongue weight. Your tongue weight should be 7-10% of the entire load. If you have a single axle trailer now, you can plan on spending 1500-2000 to add a second axle. (new tires and wheels, new axle, new springs and hardware, new fender, new step plates, possible new undercarriage, brakes, etc. plus all the labor). You might be better off just buying a new trailer at that point.
So to accomodate the weight of your boat and trailer, you will need to either goto a tandem axle setup with 2-3500# axles or step up to a single 5200# axle. But to answer your question tandem axle trailers are nice and they definitely tow better.