At the transfiguration, it was Moses and Elijah.
John is told in Revelation that he would "prophesy again."
It is given unto man once to die, and after this the judgment. The 2 witnesses are killed, so they'd have to be killable by humans.
And perhaps that call to "come up here" is the rapture, not just for those dead ones, but also the promised rapture for all of us Christians. (I think the mid-tribbers believe this.)
Very true, but does that mean that Lazarus is still around, doomed to immortality until the Second Coming? Probably not (though I've seen that idea made into good fiction), and where there is one exception to a general rule, there can certainly be two.
Personally, I lean towards the second witness being Moses, but I would be neither surprised nor disappointed if it turns out to be Enoch instead.