The first one is correct. The second one can also be correct. If "something's happening right now," the event may continue over a period of time. For example, I might watch a meteor shower over the course of an hour. But after only 15 minutes, I may already feel that I have seen and experienced it. So even though the meteor shower is still happening, I might be inclined to use the past tense forms of the verbs, and say, "I wouldn't have believed it if I hadn't seen it with my own eyes."

If it's happening RIGHT NOW then you want to say: "I wouldn't believe it if I wasn't seeing it with my own eyes." Eg. An alien spacecraft hovers overhead... "I wouldn't believe it if I wasn't seeing it with my own eyes." OR, for extra emphasis: "I would NEVER believe this if I wasn't seeing it with my own eyes." --- If something has only just happened (very recently), you could say: "I would never believe this if I hadn't JUST seen it with my own eyes!" -- If something has has happened further into the past you could say: "I would never have believed this if I hadn't seen it with my own eyes". OR, your one: "I wouldn't have believed it if I hadn't seen it with my own eyes." --- If you want to express it in the FUTURE (that you won't believe it till you see it) you could say: "I would never believe that unless I SAW IT with my own eyes". --- Hope this helps, Good luck and take care!

The tenses are actually correct in both! I think it depends on the context.

I wouldn't believe it if I didn't see it with my own eyes...this one.