Some people walk around with earphones piping music into their brain. It’s a closed system, just looping around and around/.
On the other hand, there are those who walk around piping music from their brain out into the world. Some hum, some sing and some play a harmonica while at the same time pick and strum a mandolin. It’s an open system.
Playing two instruments at once while strolling is not for those of us who have trouble walking and chewing gum at the same time.