To use any rechargeable battery like this "series-stacks-in-parallel" is ALWAYS a bad idea, due to the enormous currents these batteries WILL deliver when only a small difference in voltage between both series-stacks exists.
If I would use 2x4 eneloops and there would be 0.1V difference between two stacks, there would flow 0.5A current at max. And that is without considering the resistance on contacts. Not a big drama imho
Just always use cells (rechargeable or nonchargeable) in the same state of charge and you will be fine.