According to Einstein, if you speed up your velocity relevant to an inertial frame, you gain mass, and once your reach the speed of light, c, your mass is infinite, meaning it took infinite energy to get it to that velocity, and since there's only a finite amount of energy in the universe, conclusively, you can't go faster than c.

But I have another way of obtaining a speed >c.

Suppose two spaceships, Spaceship A and Spaceship B, start out at zero velocity relative to a common frame (say, for argument's sake, an asteroid, therefore stationary to each other. Then, all of a sudden, they take off in opposite directions (completely 180 degrees from each other). Spaceship A goes at .5c relative to the asteroid, while Spaceship B goes at .75c relative to the asteroid. Then that means, relative to one spaceship, the other spaceship is moving at 1.25c, which is FASTER THAN LIGHT!

Where am I messing up?