As an experiment, I hooked up a cheap USB 1.x non-powered hub to the USB port in the car. I was able to plug a memory stick into one of the ports, and the radio sensed it and displayed it as a source and was able to play back audio files on the memory stick.
I then tried to plugin my iPhone to the hub while the memory stick was still plugged in. It sensed that there was an iPod-like plugged in, the phone did start charging at the full 1A rate (I have USB power meter that shows the current), and on the audio source screen it showed iPod as a source and the memory stick as a source simultaneously. However, when I selected iPod, there was no music or playlists or anything listed. Also of note, normally when I have my iPhone plugged in, it shows the source as "Dave's iPhone 6s", not "iPod" so it definitely didn't like talking to the iPhone via the cheap hub. Unplugging the memory stick didn't help. It simply doesn't like talking to the iPhone via the hub, period.
Similar results when plugging in my iPad via the hub. The iPad can be plugged in directly to radio and it will treat it just like an iPhone/iPod, and will even charge it, albeit slowly, at the 1A rate that the car's USB port supports. But via the hub it wasn't usable as an audio source. It did charge via the hub at the 1A rate.
And as a final test, I tried having the iPad and iPhone both plugged in at the same time. Radio did not like that and shutdown the USB port completely. No power or data. Only way to get it back was to power off the radio and power it back on by pressing and holding the radio's power button for about 5 or 10 seconds. Basically had to reboot it.