You don't, but if both of you are holding attacks or blocks, the person moving forward only moves ~2% faster, and that's within the possible error of WaltF4's tests. All that assuming the same build and equipment. Also the speed at which you backpedal remains the same no matter what other action you are doing (blocking or attacking).
Basically in combat situations the fastest build moves faster, in any direction.
Part of footwork skill is knowing when you can release your block to go "forward" slightly faster.
