Multiplexed input is an idea from the original AJATT site. It is when you're trying to immerse with multiple sources at the same time, trying to listen to two different things. Khatz even tried to use it to immerse in two different languages at the same time. He listened to a Japanese podcast while watching a YouTube video in English.
Multiplexed input is supposed to increase density of immersion. If so, I think listening to condensed audio is a better choice. It's more organized. If you try to listen to two or more sources at the same time, if you try to understand what two or more people are saying at the same time, you're going to distract yourself from all of them. But condensed audio takes one immersion source and turns it into a stream of constant talking with no empty spaces and no distractions.
Humans can't truly perform multiple activities in parallel. But they can do them asynchronously, meaning that they quickly alternate between the tasks. So you can listen to one person talking for a few seconds and then switch to the other person, but there is no way to pay your full attention to both at the same time. In my opinion this is a very chaotic way of doing immersion, and you're unlikely to reap more language gains from it compared to condensing your audio.
So I would say that you're going to get the most benefit if you do one thing at a time and give it your full attention.
On a side note, doing passive and active immersion at the same time would be a form of multiplexed input. Doing Anki reps while listening to something in the background also falls in this category. In my mind these are bad ways to immerse because by passive immersion you're distracting yourself from the active task. Passive immersion is powerful when what you're doing actively is not related to your target language, like driving, cooking or cleaning.