Wait but when you run physics at 64Hz for a 60Hz refresh rate, you'll still see some refreshes with 1 step and some with 2, which means you need some interpolation scheme if you wanna avoid a stuttering mess still. You should have some form of synchronisation which ensures that exactly one physics step happens between each frame that's displayed to the user
You always need such interpolation nonetheless, because the physics timestep is fixed but your FPS will not always be 60 (that is, you may have slowdowns that lower your FPS temporarily)
Running two physics steps in the same frame isn't a problem (besides consuming CPU pointlessly), the problem is when you run no physics step between one frame and another
