Although I don't have a wireless groupset on my bicycle, I'm attracted to the idea of dual batteries (like in the SRAM system) - at least that way if you forget to charge a battery - you can swap them between front and rear so you have a slightly better ability to limp home.
With Di2 it kind of does the same thing, when your battery is getting low the front derailleur will stop working. Not sure how many shifts that leaves you at the back though so you might still be in trouble if you’re far from home.
The front derailleur stops working at a nominal 10% charge. You'll have plenty of rear shifting left on all but the longest rides.
That being said, it's really easy to avoid the battery getting that low. It lasts months under normal conditions. And your head unit will tell you when the battery gets low. Except Hammerhead units won't get that warning anymore? #thanksshimano.
