I believe that currently, Mbin isn't making any drastic changes, and relying mostly on Kbin's existing code as its base. As far as I'm aware, the Mbin team are mostly just doing maintenance-level development; fixing things as they break and making optimizations, but not so much in the way of developing new features. Mbin is currently just basically a copy of Kbin, without much distinguishing the two.
Since Kbin doesn't seem to be moving much at all, I think it might be a good idea for Mbin to start flexing their own muscles a bit, and making it into its own separate project. Otherwise, having a copy of a stale project just leaves you with two stale projects.