People generally think that having 2 engines is the safer option, I.e. if 1 breaks down, the other will get you home.
However, the reality is that unless you have 2 completely seperate systems (electrics, fuel, controls etc) its possible that if 1 stops so would the other.
Because of weight and drag factors you wouldnt need a 200 to replace 2 x 100, a 175 would do it or even a 150.
All my boats are single engine, I believe that as long as its looked after (usually better than my kids if im honest
) it wont give you a problem. I do however carry a tender or take a wing engine if im doing some serious offshore trips.