It looks to me like his arm is only touching a few inches of the armrest -- can't you share by putting your elbow on the armrest behind his? That's how I usually "share" armrests; I'm on the shorter side, so use the back 3-4" of the armrest for my arm, and the front part is available for the other person. If it's another shorter person next to me, perhaps their elbow goes on the back part and mine a few inches forward.