As has already been pointed out, it is possible to purchase Y cables that have two Type A connectors on one end and the desired Micro B on the other end. These cables are intended to be used on computers or powered USB hubs where the +5V connections to both type A connectors come from the same +5V power supply. The +5V and GND leads of both Type A connectors are connected together. The design intention is to get around the 500mA current limit that exists on many computer/hub Type A USB jacks that contain circuitry to enforce the load current limit out any one connector.
You do not want to try to deploy one of these type cables for your application because it would mean that you would be connecting your external power source in parallel with the +5V output of the computing device. This is something you want to avoid at all costs because one voltage source will always be a higher voltage than the other one. The one with the higher voltage will try to assume driving all the load and back feeding the other source. Bad things can happen in this situation including overload of your external supply, oscillatory behavior of the regulators on both sources as they try to fight each other or other strange noise spike behaviors.
Instead you want a cable that is built like this:
There are a couple of other factors that you need to consider as well:
The contact ratings of USB Micro B connectors may not be up to the current you are trying to put through them. So you need to check connector data sheets carefully. Take a look for example at the target device connector which solders to a circuit board of the device as in this data sheet. Notice that the contacts are only rated at 1A per contact if all contacts are used at rated capacity. In normal USB mode where the power pins are the ones carrying more current they are rated to 1.8A as long as the rest of the contacts carry less than 0.5A.
You also need to carefully consider the wire sizes in the cable to the target device. Many modern USB cables that terminate to Micro B type connectors use small guage wire to keep the cable diameter small and flexible. Small wire diameter at high currents can lead to a significant voltage drop along the cable. If the voltage drop gets too high the target device may fail to operate correctly due to the voltage drop. See the chart below for the type of voltage drop that will be seen for various wire sizes at 2A of current traveling down and back along a 2 foot cable length.