Agree with CALWATERBOY, if staying with OEM (or equivalent) doing one axle at a time is fine. Its when you change the brake performance characteristics significantly that you want to think seriously about doing both axles at the same time.
Other thing is you mentioned your car is CPO. Unless you have a significant reason to move away from OEM brakes, why give the dealer any wiggle room to deny a CPO claim in the future? i.e. if you had an issue they could blame on the brake pads or rotors if they were aftermarket. Not saying not to go aftermarket, but know why you want to.