There's a whole bunch if stuff at play here, let's say you are going from equal single HP to equal dual HP, the single is lighter, faster, cheaper to buy and maintain. The dual setup is heavier, way more torque, more stability underway, easier docking, slower, more costly in every area, offers "some" redundancy, and above all, looks good! As an example a pair of (say) 150's will pull a single 300 backwards, but the single will be faster, make sense?