Put four winters on. You're better on bald all-seasons then on only two winters. Here is a great example http://www.youtube.com/watch?v=EdtAm7RsTmE (especially the end where there show what happens with only two). Keep in mind if you have an ABS equipped car, it will be even worse. The reason being, the wheel speed sensors will be giving the ABS system incorrect information when the all-season end of the car locks up. That means that it will pulse the brakes and your stopping distance will be dramatically increased. Again, only two winters will likely be worse than bald all-seasons.
Find that hard/impossible to believe. You wouldn't be able to move, literally, with bald (well worn out) all season tires in snow. I ran snow tires in the front and all seasons (near new) in the back of a civic and it worked just fine assuming you drove carefully and responsibly. Having said that, all four tires is optimum and highly recommended. Some cars may do better with mismatched tires than others, and there are many variables that will determine that.
You might actually have a hard time finding a reputable tire dealer that will sell you only two tires. Many claim to have a "winter tire policy" where they are not allowed to sell you only two tires due to safety reasons. Not sure if there is a bylaw of some sort for that.
Also do your research before buying winter tires especially, as many have pretty poor
wet and
dry traction, which is 90% of our winter driving. If you can take transit on the snowy days, stick with your all seasons, assuming they are good quality. If you know you'll be heading into higher elevations and definitely require winter tires, spend the extra $$ and get something from Michelin, Bridgestone or go all out and get Nokians.