It feels a little like luck to me at the moment - I'm struggling to see the difference obviously enough - either because it is too fast, or because focusing on both simultaneously is difficult (it could also be being 30+). For a true test I think you'd want it to start a little slower to get more meaningful feedback, but that is just one man's opinion. I spent a good 5-10 minutes trying to hone the art but I found it quite difficult.
It's a very interesting concept and I hope it goes well for you!
If you have time - I have my own small playtest up at the moment, not expected for you to give feedback too but would be appreciated.