How does a Dual-Port Wall Charger distribute Amperage output?
My guess is that the internal power supply is connected to both ports in parallel and has a current limit. If you try to draw more than 3.4 amps in total between the two ports (doesn't make any difference how...could be 3.4A on both ports, 2A on both ports, 3.4A on one and 1A on the other, etc.), the supply will go into current limit and either the voltage will drop or the supply will shut itself off.