The gemara states (34a) that if two judges give the same reason for a judgement but from different verse, they only count as one judge. Rashi explains that this is because one of them is certainly mistaken because we do not learn two verses for one law.
Why is this different however from subsequent examples of slight disputes of laws from different verses? Can we not also be sure there that one judge is mistaken?!
Sam, Jerusalem
The difference is that if each is deriving the exact same law from a different verse, one verse is clearly not the source. However, if each is deriving from a different verse and there is a difference of opinion between them, each could be right that the verse they quote is teaching an aspect of their position. Since it is possible that each is teaching an aspect of the law, nobody is absolutely wrong.
All the best,
Yaakov Montrose