Ener You are completely right in your observation regarding the ranged attack test of injection darts; get two net hits against an armored target, and the Toxin is delivered..
Injection Darts:For use with dart guns of various types—such as the Parashield pistol and rifle—injection darts carry a single dose of a drug or toxin. Effects depend on the drug payload, but to successfully deliver the payload and penetrate armor, the attacker needs two net hits on the Ranged Combat Opposed Test.
For Narcojet, the Speed is Immediate, and the Power is 10. Now, this is important; even with a Speed of Immediate, the effects take place at the end of the combat turn.
Speed determines how soon after exposure the victim suffers the toxin’s Effect. Toxin effects are always applied at the end of a Combat Turn.
Immediate means the Effect is applied at the end of the Combat Turn the victim is exposed to the toxin.
So in the example you gave, the guard would have been acting as normal without modifiers until the end of the combat turn he was exposed to the toxin.
Only at the end of the turn is the toxin resistance test taken.
Also, depending on the guard you're not guaranteed to drop them with Narcojet.
The victim makes a resistance test using Body + the rating of any protective systems or gear. Every hit reduces the toxin’s Power by 1 point. If the Power is reduced to zero, the toxic substance takes no effect; otherwise apply the Effect depending on the remaining power level.
An average Human is BOD 3, meaning they'll get roughly 1 net hit on the resistance test. The same average human is also WIL 3, meaning they'll have a stun track of 8 + [WIL 3 / 2], rounded up, in this case 10. That means the guard has to get 0 hits on his Toxin resistance test to be instantly knocked out.
One option is of course to hit the guard twice, which would likely guarantee knockout at the end of the turn.
If a toxin is applied at concentrated levels (more than a single dose), the gamemaster may increase the Power of the toxin as he feels appropriate, as well as increasing the damage it causes or its other effects by an appropriate amount.
Note that the Power of a toxin does not double with a double dose, but instead is left up to the GM.