Just for info, at the 250 post point, the right-hand, left thumbs on top are "winning", 75 to 55. For the lefties, left on top ahead at 16 to 6.
It's an incomplete experiment. If you're just sitting there putting your hands together, get up and do it behind your back, or behind your head. I maintain the other four fingers of your hands determine where your thumbs end up, because they meet first.
Me - right-handed, left thumb on top
My husband - same
Cool experiment. Good luck, and don't forget to come back with a summary of the results!