only the wheel with the jack under it should be up in the air. If you have it jacked up long enough to paint the calipers or if you are just working near the car, seriously put it on jackstands or at least a real shop jack. Just don't leave it on the little jack that comes with the car. BTW if you do jack the car up and 2 tires are in the air the car can easily slide sideways, either landing on you or seriously damaging your car (imagine your car hitting the ground sliding sideways with no tires on)
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