Top speed is often more driven by battery voltage than Watts.
But, in terms of "get you out of the hole" acceleration, many claim dual motors gets the job done slightly better.
I am running dual 1Kw motors, about 95% of the time I have the front motor switched off, as it is excessive.
I took my bike to a skate/bike park (it had jumps, fishbowls, ramps, stairs, skids, etc). I found that the people around me were shocked to hear the sound of squealing rubber under full power; the bike has street tread for traction on hard surfaces, as versus knobby tires. The surface was cement.
I would bet that dual 750 Watt is the sweet spot for a lot of types of riding. There are times that dual 1 Kw is a bit much.