What Circuitry is needed to charge in parallel, but power in series?
Try this design, it should work for what you are trying to do:



Make sure your power supply has a regulated current output &amp; it does not exceed 4.2V.

hth, best regards!

p.s.: This small design modification will automatically switch off power to the Arduino when SW1 is switched down:



Or better yet, this design does the following:
1.  power from the charger can not be applied to the cells (as shown in the diagram)
2.  when the ganged-switches are switched to the down position, power to the Arduino is disabled &amp; power from the charger is applied to both cells in parallel.