I wonder if it can be defined through System 1 and 2 activities. So doing a System 1 task + System 2 task simultaneously would be ok, but not doing System 1 task + another System 1 task.
For example, if I'm learning to jog at a new cadence (180spm) I need to focus on just that. Once that's become my baseline I can listen to a podcast.
Similarly, if I'm at a stage of unconscious competence [1] driving to work, I can speak with a passenger. However, if it's a new section of road I'll need to pay more attention to signage, etc. at the cost of the conversation.
Instead, try to focus on yourself, your sensations, the smoothness/effectivness of your stride, cadence and so on. There is actually plenty to keep your brain busy.
It will help prevent injuries and make you a better runner.
Listening and jogging isn't, unless you struggle with some serious mobility impediment.
Sitting in a meeting is very rarely a cognitively challenging task as well.
None seem to account for: https://en.m.wikipedia.org/wiki/Context-dependent_memory
Do what works for you.
- Is sitting in a meeting, listening and taking notes multi-tasking?
- Is listening to a podcast while jogging multi-tasking?
- Cooking and talking to friends on the phone?