And yes, in a lot of cases the defense sets up the offense. And no, if looking at something like tan da, simultaneous offense/defense doesn't actually happen exactly
simultaneously, but the timing is so closely connected together that the term is safe to apply. Anyone that argues the they aren't simultaneous is just arguing semantics.
But there are different actions that do both at once in one single motion/move, like a 2-line punch/wu sau cutting into an attackers punch. That's one beat, one move, and simultaneous offense defense at the exact same time!