The two witness rule has a couple of problems. One, eyewitness testimony is easily the least reliable form of evidence. Two, it is laughably easy to abuse, thus derailing the very justice it is supposed to serve.
I mean, it's fine for old-time religious investigations, like trying to determine if little Johnny has been whacking off to porn mags when he thinks no one is watching. It sucks for pretty much anything else. It's not a substitute for a forensic investigation in any case that actually matters.