10 volt power source for field recording gear from two 5 volt usb lithium battery packs?
Technically speaking, two floating packs of batteries should work with no problem.

However, these charging packs use buck or boost converters (depending how many Li-Ion cells the pack is constructed from) to make 5V output on each port.

The problem will arise because these packs are somewhat intelligent, so they will shut down the port power if they sense that the current taken from them is too low. At least this was my experience.

Practically, if, as you say, "the device draws little power", your arrangement will not work.