I would say "I wouldn't believe it if I hadn't (just) seen it".
It just happened, so you are talking about your state of belief now. So you want present tense in the consequent: "I wouldn't believe it". In English you generally use a past tense for the dependent clause with "if" when you are talking hypothetically, even although the event was not really in the past. If you use a simple past, as in your first example, then you are talking about what might happen now or in the future, but it hasn't happened yet. But in your case it has, so you shift it further in the past by using using a pluperfect "had seen".