1. The power needed depends on the aircrat weight. A heavily loaded aircraft might just have enough power to maintain height on one engine, but it's not as simple as having twice the power.
2. If it tried to take off, the assymetric power would turn the aircraft and it would leave the runway before there was enough speed to enable the rudder to keep the aircraft straight. Every twin- or multi-engine aircraft has a "safety speed" below which it is not possible to maintain controlled flight. In the case of the Mosquito an engine failure immediately after takeoff - before reaching that speed - could be fatal.