Of course, Bernie might not make it through the first term, in which case...
A Bernie/Stein ticket would not win outright, but it would provide interesting pre-election debates, and it would block the Dems from winning. What's not to love?
Better, it might result in the death of the Democrat Party. The socialist true-believers end up in the Green Party, and all the D's have left is the Friends of Goldman Sachs and the establishment RINOs from the GOP -- not a vote-inspiring combination.
Bernie, please zot the Democrat Party!