According to Derek Muller from Veritasium, measuring the one-direction speed of light requires synchronised clocks, and that's a problem because moving one of the clocks will slow down its time. In the example from the video, he keeps one of the clocks stationary while moving the other one 1km.
While watching the explanation I though "why don't starting both clocks at a given position, say A, and then moving them in opposite directions at the same speed, so their time dilation will be the same, and they will end up at points B and C separated by 1km while still synchronised".
Obviously, thousands of people way more intelligent than me already thought about this problem, so the probable explanation is that instad of single-handedly solving this problem my solution has an obvious flaw, but I can't see it. What is the flaw?