I guess my point would be that the experience of the combination can be as immediate as the experience of the particular; indeed, the experience of a particular which is embedded in a larger whole involves the mental act where we have to "prescind" or "abstract out" something which only then gets our selective attention. But in the initial moment, we experience (what we'll later call) the complex. But we experience it as a simple.
Gestalt? Right?
Long before the time we're composing our thoughts here (around 1/1/2 yrs of age) we've mastered the task of seeing the whole and not fixate on the parts.
So what if we can't (or can we?) really, truly, and exactly differentiate two voices singing at the same time? We bask in the harmony and yet are able to discern individuals all the time even when they seem to compete for our attention.
Take a good listen to Lakme's
Duo des Fleurs and tell me you can't distinguish between Sabine Devieilhe (coloratura soprano) and Maienane Crebassa (mezzo-soprano) at the same time. I can.
The mind works so quickly so as to render the argument that it's impossible to hear both rather silly. That's splitting hairs to the point of red herring territory.
There is a lag in time with everything we do and yet we still catch balls, drive cars and bikes and some can even juggle. It's all done so fast that it's a non issue.
All the best,
Nonoise