Gears assemblies are kinda screwy in that they don't behave the same way as the on/off lever semantics used everywhere else (gears toggle).
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The same works for pressure plates set to creatures (use military dwarves to test). I believe it works the same for liquid plates though I haven't actually tested it properly.
I just exhaustively tested this, just to confirm it for myself.
Specific to gear assemblies, the concept of 'on' and 'off' signals do not exist like they do for other mechanically operated things (e.g. floodgates, doors, spikes). Both 'on' and 'off' signals from a pressure plate or lever are treated as 'toggle' by a gear assembly.
I have tested this with:
* Dual levers - triggered individually
* Dual pressure plates (configured for creatures) - triggered individually
* Dual pressure plates (configured for 'on' being 0-1) - triggered individually, both 'on' and 'off' signals.
* Dual pressure plates (configured for 'on' being 2-7) - triggered individually, both 'on' and 'off' signals.
I every case, the state of the gear changed when the state of one of the pressure plates or levers changed, irrespective of what it changed to.
As an additional test, I set up a situation where both liquid pressure plates (configured for 0-1 being 'on') would trigger at the same time. If both plates trigger at the same time, the gear did not appear to change state (or more likely it went through 2 state transitions to arrive at its initial state). So it's safe - you're not going to lose a signal due to timing when operating gears.
As a final test, I checked the speed of the signal from the pressure plate to the mechanism - it appears to be instant (the gear changed from disengaged to engaged in the same frame as the water on the plate dropped to 1, as expected).
Tested on 40d.