Flip. It's easy. You mean you can't??? I don't get it.
(but I'm ADHD. Oh. Right. We might be a fair portion of the population, but not "everybody" is)
I mean, I could, until I developed a hearing problem and now I can't hear anything!
Multiple theories here - I've read all of them somewhere over the years (different sources), and have seen all of them at work at one time or another...
1) Self-stimulation. I don't know about Aspies, but ADHDers need extreme stimulant - max Concerta doesn't even come close. Combine medications, caffeine, and adrenalin... and maybe we're getting close. How do you get the adrenalin going? either high risk behavior, or complex mental challenges. (so take your pick - would you rather he was listening to 3 things at once, or playing chicken on the highway?)
2) Self-protection. When you've been bullied all your life, you get really good at hearing what's being whispered at the back corner of the classroom - while the teacher is talking (and trying to be heard over the classroom radio playing background music). Do it long enough, and it not only seems normal - anything else no longer seems normal, so you re-create normal.
3) High-speed brain. As in - think of an engine revving at top speed - if it wasn't for certain safety mechanisms, the engine would fly apart. ADHD brains, when they get "in the groove" on a task (called hyper-focus), are running very high and very hot. The extra mental work of listening to multiple things is a bit like a flywheel - burns of some excess mental energy so things don't go flying apart.
I've done all 3 of the above AT THE SAME TIME.
Don't you DARE distract me or try and get my attention... I'm in another world, thank you. You can talk to me when my spaceship gets back.
Seriously? The rule shouldn't be "how many things do you have on". It should be - how effective are you being at the important task at this moment - even if the "important task" is a video game.
Teach them to recognize what they are doing, and to understand why they are doing it.
Then help them look at whether or not it is effective in this particular situation.
Harder - but just as important - is to then teach them to consider how it affects others. THIS factor shouldn't change whether or not they multi-task this way... but it may change where, when and/or how its done.