I'm finding it very difficult to rescue the damsel in distress without being seen. Even with slow time, (I don't have stop time), as soon as I dart a guard I am spotted. What tactics have other stealth players found useful for this encounter?

None. Sadly my runs where I had to avoid detection, resulted in me not helping lots of people.

I darted them from a rooftop, but pretty sure I was spotted, too.... I don't recall there being red bolts, which are
supposed to be true indication of detection, but.... hard to tell.

I choke them both without being spotted, it's more or less a question of positionning. You will want to manage to blink behind them right from the start ( if you arrived there by the alley, blink behind the cover on the right, then behind them ).
If I recall correctly, one of them will step forward. That's when you want to quickly choke the one behind, carry him ( do not let him fall on the ground, if I recall well, or it will make too much noise ), quickly crouch and put him on the ground, then choke his pal.

I can't remember if I already had the Strong Arms charm which almost render choking instant, I believe not. Still managed to get the mission done with no detection, no kill ( left the twins to Slackjaw ), and the steam achievement for robbing the safe before giving the code to SJ. Definitely got the service key as reward for saving the servant.
Alternatively you can choke the first and dart the second if you fear you won't have enough time. Anyway it's definitely doable with your bare hands and no power beside blink rank 1.