Listening to two talkers at a time is certainly doable...
As is talking whilst listening to another conversation - eg. Giving a lecture whilst eves dropping on the people talking at the back of the lecture theatre.
However, having a two way conversation with one person whilst listening to another is really hard.
Not sure why.
Listening and responding are likely pretty different mental activities
You're using one of the streams for yourself. :P
I'm a native English speaking pinball freak living as an expat in a German-speaking country, and I find myself often listening to English speakers on the train, while also carrying on a conversation in German .. and I have observed that the same part of my mind that can handle multi ball-pinball events, where during a pinball session multiple balls are in play, 'feels' active.
Its a kind of context juggling mechanism in both cases, and it feels like the same mental muscle being exercised in both cases.
I wonder if there is a worthwhile experiment to be conducted wherein an EEG'ed pinball player gets to play pinball with easy multi ball targets, while also listening to German and English speakers, and then passing a test at the end of it .. because I sure have been preparing for that kind of scenario lately ..