I can provide a little bit of info on this. The original version of MonoCross was developed by Scott Olson and Kenny Goers of ITR Mobility in St. Paul, MN. It's a cool framework. The first time another developer and I worked on an Android/iOS app side by side sharing ALL of our models and controllers and other non-view code, it blew my mind. The only divergence in our code repository was our view code. Amazeballs!
So, as far as the curent state of MonoCross, I don't know how much more work has been done or is being done by those guys. Until recently, I was working in the same office as them on a daily basis. The project was open-sourced from the get-go so that it could get picked up and run with.
From my own observations, it seems that Stuart Lodge's MvvmCross has experienced much more community momentum than MonoCross. I wouldn't expect as much commitment from the arbiters of MonoCross as I would expect from Lodge. Unless someone else comes along and throws themselves at MonoCross the way that Lodge has thrown himself at MvvmCross. Also, it seems that people into mobile dev are more fond of the two-way property-binding that the MVVM pattern provides, as opposed to the one-way model-passing navigational paradigm of MVC.
As to whether or not someone IS throwing themselves at MonoCross, I really can't say. It would be worth reaching out to Kenny Goers or Scott Olson.