Here's based on my experience of bicycling in Toronto in winter (a daily 18km / one-hour each-way commute) ...
Don't let your hands and feet (fingers and toes) get cold. They don't have a lot of fat and blood circulation and muscle (I guess they're mostly bone and tendon) so they need insulation. It's been decades since I last cross-country-skied but when I did I was wearing cross-country-ski shoes (which are designed to fit the binding of the skis), not snow boots.
Anyway to answer your question I recommend a zipper at the front.
- A base layer (I used a cotton shirt, to be honest)
- A fleecy with a zipper
- An overcoat with a zipper
My overcoat is bulky and down-filled from Mountain Equipment Coop. It has a built-in (insulated, optional) hood, and an interior elastic belt (so that warm air doesn't escape out the bottom of the coat). The point is that the coat is more than warm enough: if I close it up completely then it will keep me warm, even if I stop exercising (if I were waiting for a bus or whatever).
To be comfortable while exercising I undo the zipper[s] to let warm air spill out and away from my chest and from inside the coat (to keep my 'core' temperature cool enough). The fleecy, too, has its own zip. So I can get the right amount of heat spillage, by unzipping the coat and/or unzipping the fleecy underneath it, and varying the amount of unzippedness (throat, collar, mid-chest, diaphragm, or almost completely unzipped).
Some people will tell you that you shouldn't have a cotton T-shirt in case it gets wet. I found that as long as I unzip enough (and adjust the amount occasionally) then I don't get too hot and hardly perspire.
If the temperature is warmer e.g. -7°C or higher then it might be too warm to exercise in a feather coat even if it's unzipped; so then I'd have the same as above (i.e. gloves and fleecy) but a thinner jacket padded/quilted with artificial insulation instead of the feather coat.
The reason for wearing a fleecy underneath is probably so that I can unzip the winter coat. Where the coat is unzipped, the fleecy lets the body heat out, but still keeps the freezing air/breeze off my skin. At -20° you can't expose your skin to the air for too long even if your body core is warm enough (skin gets too cold even if the core is warm). Having a fleecy underneath is why I can afford to unzip the coat a bit.
I want to be able to adjust the insulation to match the wind and the heat-from-exercise (so that I'm always neither too hot nor too cold). As you may know it's fine to perspire when it's hot but it's better to avoid that (getting wet) when it's very cold.