
Snowblind |

For something like a sleeping target, it's reasonable to allow but by RAW the answer is no. Even if multiple people are striking the target, the all have to attack sequentially, because that's how the combat system is set up. The first that damages the target will wake them, assuming it doesn't outright eliminate them. The rest won't happen because the target is no longer helpless.
For a target that is helpless for some other reason, you could have a bunch of people coup de grace them at the "same time", but the attacks will still be resolved sequentially.

Paulicus |

Pretty much what Snowblind said. Readying wouldn't work either since it's a full-round action to Coup de Grace.
That said, it's out of combat so it's easier for the GM to adjucate some kind of bonus on the DC, or just force multiple saves. If an entire group surrounds a sleeping person without waking them they're pretty much screwed anyway. Coup de Grace DCs tend to quickly outpace fort saves as levels go up, anyway, so even one attempt is likely to succeed.
Carry around heavy/light picks for the x4 crit.

Paulicus |

I had an assassin in Way of the Wicked. At lower levels, everyone kept making their Coup de Grace saves and surviving. Eventually she figured out it was more effective to just full-attack and kill them outright.
Then she got more sneak attack dice & static damage and those CdG DCs went through the roof. Even a high-level fighter isn't making a 50-some fort save.

Kaouse |

It depends on whether or not the target still has the condition after the first Coup de Grace.
If they are sleeping, I honestly don't know if they get to automatically waken without rolling initiative, in which case, Coup de Grace away. If the target does wake up however, then the subsequent Coup de Grace attempts are invalid.
On the other hand, if the target is paralyzed or pinned, then the group can totally attempt multiple Coup de Grace attempts. Well, you know, unless the target dies first.