The example I gave was just that, an example. Maybe he’s really running 1.10 and 2.20 and your systematic error is 0.2 - 0.25s. How long is the acceleration zone for these flying sprints? Is the distance and intensity of acceleration the same for your flying 10m and flying 20m? What is this athlete’s 100m and/or 60m PB? If he was really slowing down that much in the 2nd half of a flying 20m you should see a major breakdown of form. I actually don’t think that it’s possible to run the 2nd half of a flying 20m more than 0.05s slower than the first without actively breaking (this drop-off would require the athlete to be about 10% slower at the 20m mark than he was at the 10m mark). For me, the difference between 2 consecutive 10m segments of a flying sprint is never more than 0.02s.
It still sounds like a timing error. Try timing him for 30m as well, and see if the difference between a flying 10 and flying 20 is pretty much the same as the difference between a flying 20 and a flying 30. If so, the error is in the timing. Most people will have trouble holding top speed for 30m (especially Flo-Jo :D), but there shouldn’t be too much fall-off.