Put it the other way, pedro. If A pleads guilty, never gives evidence, and has told the police in a statement that B is guilty too, do you think the prosecution should be allowed to read out A's statement as proof that B is guilty? No, I thought not ! If not, by what logic should it be evidence the other way, if A happens to have said that B is innocent?
If you wanted to have A prove B is innocent (or guilty ) you'd have to call A as a witness to say so, so that the jury can see A in person giving evidence on oath and see his evidence tested in cross-examination before them.They might think he's a shocking liar, whatever he says is the truth, or they might believe every word (or anything in between). Clients, as B, being cross-examined do sometimes blurt out 'Ask A, he'll tell you!' . Unwise. The judge will tell them to ignore that. Prosecuting counsel may be quick to enquire where A is. "He's alive and well, isn't he, as you well know ? In fact you know exactly where he is now !." Not only will the jury be told the law of evidence but, being told it, they'll then be asking "If A would say that, why haven't the defence called him before us so we can hear him say it? B must be lying to us or wrong about what A would say if he were called and asked!"
If both plead not guilty their statements to police about the other are still not evidence for or against that other. If they give evidence, then they can be asked about the truth or otherwise of what they've said about the other, as explained above. .