based on the number of times i've seen or heard ppl try it, just on personal experience, its about 1 in a million you can actually sign 2/2. at 80% to sign, its 64% to sign both, and i've seen about 15-20 (20 at 50% is 1 in a million, so just rounding, maybe its more like 1 in a couple hundred thousand, based on my observations).
but if you extrapolate to all the people who read the forums and have seen a few of the 50 or whatever posts on the subject, i think the odds are far longer than that, that you can actually do 2/2. probably at least one in a million million or more (that would just require roughly half of the example to be unknown to me, i suspect ive only heard of a much smaller % than half, which would make the odds much longer).