Indeed, that's the idea being thrown about. However, I don't think WB had the balls to actually get that plan in stone before shooting a frame of either franchise. That's going to make it damn hard to nail down when the time comes, and will take a genuine commitment and love for the franchises created by the work environment making each. But just consider the likely cast of BvS:

Christian Bale as Bruce Wayne/Batman
Matthew "because Singer pulled this out of his ass I'm now a megastar" Bomer as Superman/Clark Kent
Kevin Spacey as Lex Luthor
Michael Caine as Alfred
Gary Oldman as Commissioner Gordon
Kate Bosworth as Lois Lane
Katie Holmes as Rachel Dawes
Morgan Freeman as Lucius Fox
Frank "I can't believe 'House' wouldn't let Hugh Laurie do this, but oh well, it's my fortune" Langella as Perry White

This is the cast that would be in the picture, plus whatever member(s) of the Rogues gallery gets plucked out for Gotham's criminal element in the film. Lex Luthor mixing it up in the Rogues Gallery. Batman mixing it up with Superman. The "Worlds Finest" fan-film-trailer could actually become a reality.

Just please, no Aliens or Predators.