How can I prevent glasses fogging when cycling in the cold with face covered
What you are looking for is a product that covers your face (to keep it warm) seals along the upper edge to your face (to keep breath from leaking up under your glasses) and uses a design to direct your exhalations away from your face (so your exhaled moist breath doesn't fog your lenses).  

There are actually several products on the market designed to do these things.   Many people do cold weather outdoor activities (fat biking, skiing, snowboarding, snowshoeing, etc) have these issues and they are attempted solutions in various manners.  Chemical lens treatments, electric heat and or air movement, and design directed airflow are all out there.  I'd suggest checking with a local sports store and seeing what they support/use/recommend.