Combining two exact USB battery packs in parallel to increase current in a simple fashion?
No, you cannot just parallel two USB outputs and expect to double the current capability.
The USB battery packs work using a DC-DC invertor to provide the 5V/2A output, and if you measure the 5 V produced you will find differences between the units (just from the variations in components used).
Since one unit will have a higher voltage than the other, it will supply more current. How bad the mismatch will be is unknown.

Since both units will have a negative output voltage slope ....as load increases the 5 V will drop, it is possible that they could balance at a point.
That does give you more current than a single unit could supply, but there is no way to know when any given USB pack may shutdown, so you are unlikely to be able to produce consistent results.