Well, decide how far you want each eyebrow to go, and how fast.
Then, you can start them together, or whoever you want.
For example you could -
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Have one go all the way up, and the other half way (quizzical)
Then bring them down together. -
One up, one down, alternating (parlour trick) alternating a couple of times.
The flag simply says ‘starting from where I am (here), I want to move this far in this direction (there), but if you write the code differently, you can do this a completely different way.
Create a function that simply takes the target position.
Keep a static (i.e. persistent) current position - then the function simply manages the run from ‘here’ to ‘there’.
For best effect, use a millis() ticker to update the position steps, then you have control over speed, and other stuff can happen at the same time!