If you choose the sneak option, you MUST fight the Khans since you will anger them the moment you untie the hostages OR as soon as you attack the Khan that is guarding them. The only thing the sneak option does is allow you to sneak up close to the Khans before they attack and the fighting starts. If they spot you with a weapon drawn, at any time, they will attack. They won't attack if you go in with the negotiate option. But even then, you can only negotiate a hostage release, you can not release them (untie them) yourself or else fighting will commence. These are just my observations in multiple run-throughs.
This, basically. As far as I know the only option to save the Khans is to talk it out with them. Sneaking and fighting lead to the same end.
Bear in mind the only reason the NCR haven't attacked is the Khans have their men hostage, once the lad and the lass are freed, they have no inhibitions, their orders are to kill the Khans. (Seriously, the NCR have got some mega beef with those guys...)