I don't know because the story says that they separated because the two founders of Unova fighted about the way the region should work.
So how does these formes of kyurem fit into the legend, it is all a bit strange to me that the legend doesn't mention these two pokemon at all (and as serebii stated in the news post, masuda listed them as separate pokemon).
Bringing myself back to the dual legend point I made. Maybe the separation was of Zekrom and Kyurem and Reshiam and Zyurem, and infact this is another part of a dual legend which has somehow made itself into one.