Recently there has been a ton of speculation that Batman/Superman would just end up becoming Justice League, but it turns out there may be a little more to it.
According to a recent article from Latino-Review DC/WB have decided to film both films (Batman/Superman and Justice League) back to back to not only cut costs but to maximize everyone’s schedules and get the movies out as quickly and efficiently as possible.
They go on to indicate that all the recent casting rumors for Wonder Woman, Nightwing, Flash, Green Lantern, etc wont actually be full on roles in the Batman/Superman film but rather a big group cameo at the end of the film leading into the Justice League that they hope to put out the following summer in 2016.
Until we see more concrete confirmation on this, right now we can put this over in the rumor section, but it just may turn out to be the winning pitch for WB to do things this way.
Batman/Superman is currently slated for Summer of 2015 release.