Lithium Ion Cell balancing
It's likely that balancing will be reasonably OK over a moderate number of cycles. You can measure the two cells occasionally and if there is a significant imbalance, discharge the higher voltage cell &quot;enough&quot;.

A simplistic method that will work well enough is to place a 4.2xV clamp regulator across each cell. I say 4.2X rather than 4.2V as ideally it will be just above what the regulator sees as Vmax/2.
The regulator needs to be reasonably precise. A cheap and easy version is a TL431 shunt regulator diode with a Series resistor load in the Cathode to V+ lead and two resistors setting the voltage (Vref to ground and Vref to V+).
Setting the trip voltage to 4.25V at 25 degrees C should be &quot;good enough&quot;.
TL431 Datasheet
Figs 21 27 30 relate to this application.
A TL431 by itself has limited sink current.
Fig 30 shows how to easily and cheaply extend the current capability.
If doing this, use of the 0.5% part is recommended plus 1% (or better) voltage setting resistors.

http://www.baronerosso.it/modellismo_articoli/show/622/lipo-balancer.html

An implementation of the above concept.
From here


And again ...

Here is a simple method that should be effective.
From here
Note that this loads the battery at all times when powered on.


A slightly tidier implementation - but probably no more effective. Allows higher rebalancing current due to transistor buffer.
From here