As others have mentioned, my guess is that they will treat it as disbanding the household account. When that happens, each memeber leaves with the miles they actually earned. When you log onto BA.com to view your miles, you see your own individual miles earned and a separate line showing the household total. That will give you an indication on how much you will leave with.

Now if the divorce courts rule that it must be split in half, then I guess BA has to figure out a way to do it.