The boat with two motors

I do not think that the $P \propto v^2$ relationship holds for boats. It is generally agreed that at low speeds it is more like $P \propto v^3$ . At higher speeds it becomes really tricky, and depends on the design of the boat. If the boat is not designed to plane (skim on the top of the), the power needs to grow much faster than $P \propto v^3$ above a certain speed that is determined by the shape of the hull. If the boat is able to skim, much less power is needed.

The speed is determined by the power of the motor. When both motors are active their power sums up. The power (the energy per unit time, $P$ ) is proportional to $v^2$ , so we can say $P_1 = \alpha v_1^2$ for the first motor, and $P_2 = \alpha v_2^2$ for the second.

$\alpha (v_1^2 + v_2^2) = P_1+P_2 = P = \alpha v^2$ , or $v = \sqrt{v_1^2 + v_2^2} = \sqrt{5^2+10^2} \approx 11.18$