Discount Tire has this in their FAQ:
"Replacing Two Tires
We recommend that you replace all four tires at the same time. However, we understand that sometimes you may have to replace only two tires at a time.
If you’re only replacing two tires, we recommend mounting the new tires on the rear axle, which adds traction and increased overall safety to your vehicle. Your car can better resist hydroplaning with new rear tires, even if the front tires are worn. If the front tires hydroplane, the vehicle will travel in a straight line and you can safely maintain control. While you steer, the unworn rear tires will help stabilize the rear of the vehicle.
Placing your new tires on the front of the vehicle creates a less manageable driving situation. When worn rear tires lose grip on wet roads and hydroplane, the vehicle’s rear end can swing out of control. Steering or breaking will not stop the rear end from fishtailing, which could result in a total loss of vehicle control."