my gifted DH has the opposite problem...I talk to him, or ask him a question, and after several minutes of me blathering on (or silence on his end) I realize he was somewhere in space (literally, he's an astrophysicist, lol) I often find myself having to repeat things or stopping to 'check' if he's tuned in. Talk about frustrating! Maybe you simultaneous conversation havers could give him a little advice so his DW doesn't brain him with his laptop??
As for DS4, I see a little of both sides. He can be so immersed in something that it's hard to pull him out of it - other times, he has picked on stuff that was said while he was clearly playing/focusing on something else...which, early on, made us wary to talk about him (or things that aren't necessarily appropriate for him) in front of him, regardless of what he is doing.
I wonder if there is a gender bias here...do you think females are better at processing multiple conversations in the way that they are (anecdotally) able to multi-task?