I have two USB power banks that according to USB standard supply 5 V. I want to power my EV3 brick using my two USB power banks wired in series and to the EV3 battery slot.
Can I power supply an EV3 hub through the battery slot with 10 V?
Don't ever put power-banks in series or paralel !!! They are not plain batteries.
Unless you are an electrical engineer that understands the exact circuitry inside the power-banks this is in general NOT safe.
Wiring multiple power-banks together can lead to serious problems. Like premature wear on the power-banks or even damaging them completely (as in "overheating until they catch fire").
And, in this case you don't need to as David already said in his answer. One should suffice.
Yes. The EV3 technically only needs 5V to run. And just the EV3 by itself pulls around 200mA at that voltage. However, motors could be a problem. They can pull up to 1A when stalled, which could overload the power supply. Without knowing the technical specifications of the actual power supply, it is hard to say what it is actually capable of.