[dogecoinbase]

if a Turing machine were a physical object ... either one of those moves completes before the other, or both complete simultaneously.

...depending on how fast you're moving relative to the machine heads and in what direction. Simultaneity does not exist in the real world.

[argv_empty]

We use the tape's reference frame, of course! Looking at data stored on some particular device in the physical system is going to force a particular reference frame, where you will have a defined simultaneity. A global definition of simultaneity is only really needed if your system promises to provide sequential consistency.