You're right in that humans cannot multi-task, but we can task-switch rapidly. And that is, I think, the only way to avoid the tunneling you're describing.
The only way I know of to practice this is (as others have mentioned) to incorporate random outside events into your drills. Like having a spectator randomly smack you in the head.
The problem with this is that your mind then learns to task-switch when you're drilling, but not necessarily other times.
I guess we could all go the Inspector Clouseau route, and hire someone to randomly attack us, but that seems impractical.
The only way I know of to practice this is (as others have mentioned) to incorporate random outside events into your drills. Like having a spectator randomly smack you in the head.
The problem with this is that your mind then learns to task-switch when you're drilling, but not necessarily other times.
I guess we could all go the Inspector Clouseau route, and hire someone to randomly attack us, but that seems impractical.