It is recommended that you replace all four tyres of your vehicles at once. However, if you are only buying two tyres, then it is recommended that you mount them on the rear for safety.
New tyres mounted on the front- The worn tyres have less tread and lower hydroplaning resistance, and hence can cause the rear end of the vehicle to swing out of control.
New tyres mounted on the rear- Newer tyres have a deeper tread and a higher hydroplaning resistance, which helps the tyres grip wet roads. The driver can steer to compensate for the lowered traction of worn tyres on wet roads, thus maintaining control.