Some tire suppliers would tell you that if you put just two tires on your car that you are creating a vehicle split personality. What they mean is that you put two snow tires on the rear and leave regular tires on the front. This would give you great traction in the snow from the rear of the car but not in the front, giving it a different characteristic in the rear than in the front.
I recommend replacing all four tires at the same time if you can afford to. The reason is in most cases the tires that are on your car are most likely all the same age. Although you may not be able to recognize it, they are all worn in one manner or the other. Rubber goes bad over time and even quicker when exposed to the sun's UV rays. So it is a good idea to replace your tires when needed.
Of course there are some exceptions, like when your tires have less than 5,000 miles on them and you have a blow out. You are absolutely not going to buy four new tires. One tire would be fine at this point. Just use good judgement and you'll do fine.