I have twin yamaha 4 stroke 115's on the back of my 7.5 tornado.. altho its a lot of horse power I only get about 40-42 knots... so if its speed you are after, I'd say go with a single engine as other 7 meter ish rib get better acceleration/top speed than mine - i.e Richard's Blue ICE and Rouge Waves Scorpian.
However if its safety you want, two engines is very nice, I have two completely seperate systems - 2 fuel tanks, 2 batteries, 2 fuel filters, 2 ignitions... so if a failure was to happen (like when I had water in one of my fuel tanks and the engine wouldnt start) you can still get home safely without having to call the coast guard etc.