When i bought mine in May last year it was 1 usb was standard and 2 if you had the optional sat nav fitted. The second usb is more for installing updates/risk areas/peugeot connect apps for the sat nav.
The second USB is mostly for cars that have SatNav. You use one USB for 3G Dongle for internet application, and the second for USB to play your mp3's.There is no way to make them function together as mp3 players, as there is a USB hub in middle.
But you can use one to power a phone for example and other to listen music.
Also can use one with Ipod, and other with USB containing mp3.