There is no difference between you moving with velocity X and the other party moving with velocity -X; or you moving with velocity 2X and the other party being

There is no difference between you moving with velocity X and the other party moving with velocity -X; or you moving with velocity 2X and the other party being stationary; or you being stationary and the other party moving with velocity -2X. There is no experiment you can perform that will tell the difference.