While it is always best to replace all 4 tires at once, we sometimes replace only 2 tires due to a blow out or unevenly worn tires. When replacing only two tires we always put the new tires on the rear and the old tires on the front. While this may seem counter-intuitive, please see the attached Pep Boys video for further explanation and illustration of what happens to the handling of your car when the new tires are installed on the front.
Video source: pepboys.com