It's basic deductive logic
If you're giving an argument, then is the claim that you've made really self-evident?
Person A makes a mistake X% of the time when making a binary decision (guilty/not guilty)
Person B makes a mistake Y% of the time
Therefore, the odds that person A and B will both make a mistake on the decision (both be wrong) is necessarily going to be lower than both X% and Y%
I don't see how your conclusion follows:
Premise: Person A makes a mistake 1 in 4 times.
Premise: Person B makes a mistake 1 in 4 times.
Conclusion: Chances of A and B, making a decision together, making a mistake is 1 in 16.
Is this something like what you're saying?
If so, then you're just wrong. For one thing, the conclusion doesn't follow from the premises. You would be correct if they were each making a decision about the same thing independently of each other (just as my chance of rolling a 1 jumps from 1 in 6 to 1 in 3 if I roll two dice instead of one). The chances of them both making a mistake, independently of each other, would be in 1 in 16. Nonetheless, even then, your overall conclusion still doesn't follow. If each makes a mistake 1 in 4 times, and they're both making a decision, the chance of at least one of them making a mistake actually doubles. The chance that at least one of them will fall into error is actually 1 in 2 instead of 1 in 4, if they're making a decision independently of each other. At any rate, your argument simply doesn't follow at all if they are making a single decision together.
Second, I once remember reading or hearing that, psychological/sociological studies (one of the two) have shown that people actually exercise greater prudence, are more careful, etc. when they have to make a decision all by themselves. "Group think," since it strips the individuals of individual responsibility, makes the individuals more careless.
Consider:
Someone asks me a question. Only I am allowed to answer, and it's important that I get the answer right. I am more likely to think carefully about it and try to arrive at the best answer I can.
Someone asks me a question, and there are 5 other people I can consult with. They all say X. Whether or not X is true, I can say: "Well, they're all saying so. Why not? I'll just go with that." Why put in so much mental effort when there's no personal responsibility, when I can just "pass the buck" onto the group collectively? Not to mention, of course, that it's never fun to disagree with an entire group of people, right? That makes you an outsider, and that's bad, right?