How to Power 2 devices at different voltages with the same set of power source(batteries)
No. Your attempt to power a 4n load, and and 2n load from balanced batteries, shorts out each 2n battery.

You have two (or three) options.

a) Power the 4n load, and a 4n->2n converter to power the 2n load. This could be a linear regulator if the power level is low enough for you to not worry about the inefficiency and heat. Better though would be a buck regulator.

b) Configure the batteries as 2n, and power the 2n load, and a 2n->4n boost converter. Choose (a) or (b) depending on which load draws more power, or the convenience of having a buck or boost converter to hand.

c) Power the 2n load from a battery tap, and tolerate the consequences of the imbalance (replacing batteries earlier, or charging differently)

d) Power the 2n load from a switchable battery tap, so you spend 50% of the time powered from the 'top' batteries, and 50% of the time from the 'bottom' ones. Obviously you cannot common the grounds if using it like this. 

e) Use a 2n set and a 4n set of batteries. You need more batteries, but get complete freedom.

Ok, so 5 options.