Both are correct. "... we might never have met" is recommended. As Jeremy mentioned, adverbs of frequency go after the first verb when there are multiple verbs. See English Grammar in Use 4ed unit 110C.

I agree with Jeremy.

First the dry, complicated answer. The placement of adverbs of frequency, in this case "never"(and many other types of adverbs) is immediately after the first verb in a multi-verb verb phrase, in other words, immediately after the auxiliary verb. might never have met. Now the more colloquial answer. They both convey the exact same meaning and both are quite nice sounding sentences. The first version sounds, to me, a little bit more educated while the second example is more likely what you would hear in real life. It would most likely be said as "If I hadn't missed that train we might've never met."

we might have

I would say the second one as it sounds a bit more natural to me. I don’t know if the first one is wrong per say but I wouldn’t say it that way.