A frequently asked question on BMW forums is in regards to replacing run-flat tires with conventional non-run-flat tires. There is much debate about this topic. In fact, I have a blog post titled "Ditching Your Run-Flat Tires for Non-Run-Flat Tires on Your BMW? Read This First!" that you might want to check out if you're considering making the switch to non-run-flat tires.
What should you do if you only need two tires? Can you replace just two tires with non-run-flat options while keeping the other two run-flat tires on the vehicle? The best advice I can give is to avoid mixing run-flat tires and non-run-flat tires. The consensus between tire manufacturers, automobile manufacturers, customer feedback and real life experience is that mixing run-flat tires with non-run-flat tires often causes very unstable handling. This is especially true at highway speeds and when making turns with the vehicle, even at moderate speeds.
Why does this occur? Most run-flats have stiffer sidewall construction to maintain stability during an air pressure loss. Stiff sidewall designs are what primarily give run-flat tires their quick steering response and firmer ride. Installing tires on the front of a vehicle that has handling characteristics very different from the rear tires results in a vehicle that feels loose and not planted to the road. This is similar to a person trying to jog a few miles while wearing a snow boot on one foot and a tennis shoe on the other.
For further information, read "Mixing Tires."