For all intents and purposes there is no difference in battery life between a Series and Parallel mod.
The basic math equation is The Wh (Watt Hours) = mAh × V / 1000
In series:
Wh = 2000mah x 8.4/1000. = 16.8 Wh
In parallel:
Wh = 4000mah x 4.2/1000 = 16.8 Wh
Two identical batteries will have the exact same number of Watt Hours regardless of Series or Parallel configuration.
Now, in practice, Series is slightly more efficient than Parallel, but the difference is tiny, maybe a few extra puffs from a Series mod. Nothing you're likely to notice. Chip efficiency will make a bigger difference than Series vs Parallel.