1) If one assumes the VP pick actually matters at all, it's only in terms of optics and swing states. Something neither offers.
2) I don't think they actually like each other at all regardless of what they say.
3) I don't think the general election will be won by appealing to the left and the further left in the way the primary process forces candidates to do.
4) There are multiple better picks for VP.
Yeah, adding a socialist/atheist to the ticket to energize the base doesn't seem wise, especially when they're both white/old/not from swing states.