Two ports of the same PowerBank on series
No.

The PSU will most likely have one voltage regulator giving out 5 V. This is split to feed two USB sockets, each with a connection to +5 and GND.

If you try to series connect these you will connect the +5 of one to the GND of the other and short-circuit the supply. If the thing has proper individual current limiting on each port then the other port may continue to give out 5 V. Otherwise the regulator will go into current limit and shut down.