How to get double the speed from 2 regulated EV3 motors?
By adding two motors together with gears, you have doubled the torque the motors provide. Power = torque x speed, so if you don't want the doubled power from the motors to go to the torque component but to the speed component instead, I think you should use a differential and put the motors on both outputs of the differential, and use the input to feed your construction. I haven't tested this out though.

EDIT: Here is a picture I found that shows how 4 motors can be joined into a single output with 5 different motor combinations (and many more different speed combinations):