This device has two USB ports, one port is 1.2A and follows the official "USB Battery Charging Spec". This port can be used for quickly charging most Android phones, or other devices compatible with the official spec.
The second port, marked "Fast Charging" however uses the incompatible proprietary Apple charging method. If you connect a non-apple device to this port, it will only draw 0.5A at most, which is not enough for most devices to charge and be used at the same time.
This is very confusing as the TomTom description implies that this port will work with any high power USB device, which simply is not true.
Just wanted to add 2 comments,
1) You can trick a compliant device into drawing more then 0.5A on the "Fast Charging" port by using a special "charging-only" USB cable or adapter. So NOT a regular USB data cable! Such a charging-only cable has the 2 data pins (D- and D+) crossed which will cause the device to draw more power.
2) I just got a new car, a 2012 BMW 3-series (F30) and the charger will not fit with USB cables attached in the centre console.