I'm not sure I understood the last half of that, DB, so if my reply makes no sense, un-confuse me.
So, with that position in the video, imagine the guy gave a jab right and followed immediately with a hard left while you were fading off the first one. He reaches a bit (not much required), and the right isn't ready to fire again yet. Here, you have a bit of space to use a 2-hand response like this. He's showing it without footwork, so it looks worse than it probably would with some movement. It's also way out of context, so I can't tell his awareness of the other arm. If I come into the arm with both hands that way (facing the arm, from the inside), I messed up and am attacking that arm and shoulder/neck HARD to try to reduce impact from the hand I can't defend against. When we use a 2-hand response, it comes off defending both sides: one hand blocking/handling the primary attack, while the other "occupies space" to be available to defend the second if it shows up. I've seen it taught to beginners with both hands on the one arm like that, and corrected later as they get the movement down. It's usually done to make sure both hands stay up (a problem for beginners), but I prefer to teach the separate jobs from the start.
EDIT: I just understood the last part, and he's going to need that (I call it an "arm helmet" block).