This topic will generate various opinions because like ....noses....everyone has an opinion on it.
I have Labs. I feed twice a day. They get half their ration at ~ 7AM and the other half at 5PM. These times can vary during hunting season.
As to my view on the 2x, it is simply a fact that Labs are rather high up on the list of dog breeds subject to GDV (Gastric Distention and Volvulus = stomach twist or torsion which is potentially a fatal problem.) A family member lost a truly spectacular German Shorthair to GDV; it's a very ugly experience.
As a view towards preventing GDV:
http://avetsguidetolife.blogspot.com/2011/02/preventing-bloat.html
Now, as I said, 2X is just my opinion. 2X has never been an inconvenience for me as the non-hunting season times coincide with my daily schedule. During the season, the dogs always get fed at least 3-5 hours prior to shooting time in SD, about 2+ hours prior in "sun up" shooting states like KS. I'm up getting ready anyway and they are only getting a 1/2 ration. This is a rather light stomach load of a quality food, for my small dogs as little as 3/4 of a cup, 1 cup for the bigger male. I think this minimizes my chance of GDV. In my 35+ years with many Labs, I've never had a GDV problem doing it this way.
I have also never found a need to feed anything else during the hunting day to keep the dogs energy up. I haven't had to use "performance bars", etc. Mostly, they just need to be kept hydrated which is why there is a 5 gallon jerry can of water in my truck.
Now your mileage may vary and I respect the opinion of once a day feeders. Do what works for you. Certainly a dog can make it once a day feeding, no question.
There's a lot of conflicting information out there on this topic and like most things, people do what make sense to them.