Electrical – Connect two mobile power banks in series

I have two 20.000 mAh mobile power banks with two USB ports that can deliver 5V at 2.4A. I need a minimum of 9V at 2A in output.

I have 2 male USB connector cables and I want to connect the negative wire of one power bank port with positive wire of the other power bank port (not the same power bank port obviously.)

I have done a try, and it seems to work properly, I've powered a device with 10V by wiring in series two power banks.

I did another test, by connecting the two power banks in series, again, and switching off one of them.

Unexpectedly, the circuit was not interrupted, but I got 5V (instead of 10,) so in this way the current traverses the turned off power bank in reverse (+ flow in – wire of the second power bank, and exit by + wire?)
Nothing was damaged, but can this be a problem?

I know that every current source can be wired in series to get a higher voltage, and the maximum current can be the same as the lowest current generator.
Basically, every power bank has one or more 3,7V cells with a boost converter to convert 3,7V DC to 5V DC.

P.S. I know, I can simply use a QuickCharge power bank to trigger a higher voltage, but I can't in this case for some reasons.