Running 2 5V USB battery power packs, Can I use them in parallel
I would highly recommend against paralleling two of the typical "power bank" devices. A far better strategy is to use one till it is depleted and then unplug it and switch over to the other one.

There are a good number of reasons for not doing this:

1) Many power banks use voltage converter circuits of questionable design. I've tested a few that cannot even output 5V unless they have no load and under even moderate load will drop to even as low as 4.6V.

2) Power banks are highly likely to output different voltages that vary with the amount of charge left in their internal batteries. This is due in part to the crappy circuit design as mentioned above. When the voltages are not the same on the outputs then trying to parallel the outputs will lead to problems. The least of which could be that one unit supplies all the load till it has depleted more than the others.

3) It is never a good idea to parallel up the outputs of switching power supplies. The behavior of one in trying to adjust the output voltage could interact with the other and even lead to a detrimental oscillatory behavior. Only parallel supplies that are manufacturer specified by design to support parallel operation. The most robust of these have a shared sense line that strings between the paralleled supplies to make them all play nice with each other. Power banks, which do contain switching power outputs, most certainly do not have that sharing capability.