They shouldn't do, you must be approaching too noisily or something.
Usually when there are two guards together, they have patrol patterns and one tends to stay slightly behind the other when they both stop. If you grab one from behind the one furthest ahead won't realise
Unless you time it badly.
I'll record some footage if you like. I know what stealth walking is (you move slower than full speed walking/crouch walking).
100% of the time if I hold up the guy behind, the guy in front is alerted to my presence.
I'd assumed it's because he hears me telling the other guy to put his hands up. I mean, he practically shouts "Don't shoot!" when you do it. So not hard to hear this going on.
Are you sure you've done a hold up with two guys close together like this? Because I've attempted it about 5 or 6 times, with the same result every time.