So even if the electricity goes out, the house is very warm, even without the fan?
Yes, about 80% of the heat goes up the flue without an insert.
my question is....with a wood stove downstairs, can the upstairs fireplace (currently with a gas fireplace that works poorly) also have a wood insert?....does two fireplaces mean two chimneys?
Depends on the scale of your house and how you regulate air currents. Our stove keeps what I call “the core”of our home warm. It’s a small Cape Cod, but we have doors on non-essential rooms and to our upstairs.
We lost power for 3 days in an ice storm a couple years back and we stuck it out and kept pretty cozy. Though we did have to sleep in the living room. So, yes if you lose power you stay warm. Our 2 bathrooms are included in “the core” and we did not have an issue with water freezing during the lenght of our 3 day stay with nighttime temps in the 20s outside.