That's not exactly true. There are three options for the judge to consider:
1) Officers are always trustworthy and I actually committed the offense. However, if officers are trustworthy then we have an irreconcilable conflict with the two officers. Logically at least one must be incorrect, so no matter what this option provides no solution. If you choose to 'cancel out' the two officers then I'm the only evidence remaining, and so by preponderance standards I should win the verdict.
2) Officers are trustworthy but human and make mistakes. By this option either the citing officer or my witness officer made a mistake. This SHOULD come down to who is more likely in error - the one in the car with me from the beginning, or the one who managed to supposedly catch a glimpse of a seatbelt on a passenger in a moving car traveling perpendicularly to his own vehicle through traffic well ahead of him. It SHOULD result in a judgment in my favor.
3) Officers are not inherently trustworthy so there's no defensible evidence that I committed the offense. In this case, obviously, there wouldn't even BE a citation system because if officers are no more trustworthy than citizens it's usually going to come down to he said/she said. In this case, however, I have more people on my side, giving me the preponderance of evidence regardless.