It all depends on each hull. You are increasing the weight issue but at the same time the power. So the hull is what makes the difference weather you will be faster or not. Generally twins are on larger displacement hulls. But you can achieve the same top speed with a good single depending on the twins power. An example would be twin 200 or 225,s on a 21ft skater. A single 300 can spin a bigger wheel than the 225,s will thus giving you more top speed. The ratio of weight with the twins to the single would be close to 400lb difference. Even with twins you would be hard pressed stock to run anything larger than a 30 pitch with efficiency. The 300PM on the other hand could spin a 32 cleaver quite easily. Twin 300,s would be overkill and the weight to great but people do it? Once you start getting into the 24ft and up range twins make more sense. some just like the extra safety of having a back up engine when off shore. The displacement is the key factor for twins. If the hull can handle that much weight off the transom great. On some hulls it also gives you better stability at speed. Theres 100 more reasons for or against. Generally its the size and the purpose. You will not achieve less wetted surface if the aft portion of the hull is to heavy for that hull with twins or even a single for that matter. You need the beam like 7 to 8ft+ for twins. Single faster on less than 21fters and twins faster on 24ft+.
