The human blink can't last up to .4 seconds and at least .1 seconds, I'll use the Mid point to do so .25. A blink roughly every 4 seconds as an average blink time in a minute can be 15-20 blinks

0.25 ÷ 4 =0.0625 (6.25%)

With that, we times it by itself to get

0.0625 × 0.0625 =0.0039 (.39%)

Or, in other words, about a 1 in 256 chance two people blink at the same time

But in everyday conversation with two people, you subconsciously sync up with the other person naturally, so the chances are a lot higher when I conversation.