why do you want leather shells?
As TomD said - you remove the outer mitt for a while then put it back on.
A mitt that allows you to put your thumb in direct contact with your fingers is a good idea.
I was just given some mil-extreme cold weather insulated mitts - leather with sheepskin backs for warming the face - nice - heavy.
I have some kombi goretex shelled insulated ski gloves [yes gloves] that have served me well at -5F.
Jim