Backstab and perception Options

[pster]

I've got this scenario where I'm ambushing 2 guards patrolling. I roll infiltrate to sneak up to them from behind vs their hearing perception right? And lets say I beat that. Lets say one guard is a little more further back like 3-5 metres behind the other guard. I go up to the one furthest back and backstab him. Everyone rolls a surprise test roll right? I get the +6 dice from ambush. What I'm asking is will the guard further ahead notice me? Lets say I cover the guy's mouth with my hand and pierce a knife from behind. Does his friend automatically notice me since we already are rolling initiative for combat or would his friend get to roll another perception check threshold 3 for muffled. So in the event he failed that perception test again, I get to surprise him in the 2nd pass if he also failed the initial surprise test earlier. Since he did not manage to perceive that his friend was killed.