Playing devil's advocate...

Malalaman lang natin 'yan kung magkakasubukan kayo, but with some sort of safety note, you can do it one at a time. He drives, you as a passenger and a timer on his car, then same setup in your car... then swap kayo kaw naman driver, siya timer... this way it will somehow eliminates variation and driving style factor. Where? Hmmm, that I don't know.