Can I join the two usb outs from the batteries in series to make 10 volts? This is not running from a computer and I will charge each pack separately. The device draws very little power but needs 9-16 volts to run. I will be using a hirose connector at the device end and a fabricated y cable with dual usb (male) plugs that are wired in series? Possible?
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.
I agree with the other answer about possibly needing a minimum load (easy to fix: add more load!)
But the bigger concern is safety. The unsafe part comes when the power packs are nearly depleted. One pack will shut off before the other once since they are not coordinated.
Let's say PACK1 shuts down and PACK2 is still running. Then it looks like this:
PACK1 now has 0V but PACK2 is still trying to shove current through it. The bad part is that PACK1 is applying current into the - side and out the + side of PACK2. This means PACK2 is applying a reverse voltage to to PACK1!
If enough current flows (e.g. the load is low enough), this can damage PACK1 and may cause some dangerous internal condition.
If your load is a smart electronic device such that it is guaranteed to disconnect when the voltage is too low (e.g. < 8 or 9V) with no residual current, then it might work fine. The shutdown pack would see only a small reverse voltage and little to no current flow.
If the load is some light bulb, motor, or something where 5V will continue to flow, then you have a high potential for a safety or damage problem.