Totally agree with Grey. If you take your current motor and add a second one, it will not be any faster. That is unless aerodynamics or drivetrain friction is holding you back from reaching the max RPM of your motor with your current voltage (note: your motor will likely be overheating already if this is the case). But it can be a useful thing to do in the right situation. Let's say you want a certain RPM out of your motor to reach a certain top speed. If you run a motor in an RC that has way too high a KV, it will put a strain on the motor, causing it to overheat. Adding a second motor will share the load and allow you to reach a much higher RPM. So technically no, it will not make your RC faster by just adding a second motor. But running dual motors will allow you to run much faster motors and or taller gearing.