voltage output on double USB chargers
If there is a single label, it indicates the output of the whole charger. Hi-power chargers usually also specify max current per port because a USB connector has its own limit. 4.8 A may be too much for a single USB port even if the adapter can provide such current. Normally, current of a standard USB port should not exceed ~2A, see What is the maximum physical current that can pass through a standard USB cable?

Speed of charging will also depend on the quality and state of the cable and connectors, and whether the device and the charger both support a fast-charging data protocol.

Ideally, if the total current draw does not exceed the limit, multiple connected devices should not affect each other.