Two “1.5 V” cells work with some red LEDs, or maybe with XM-Ls because of their low forward voltage.
Usually it requires a boost driver. I have a nice new 2 x AA light that is a small zoomy pill in a very nice inexpensive body. It is not much brighter in 2 x AA form than it is in 1 x AA form, but the run time is more than twice as long.
I think the greatest appeal of 2AA lights is their decent run times and universal availability of batteries for the non-flashaholic.
In addition to having numerous 18650 and 26650 lights (including two S-20 Green Goblins) I have 2-AA lights tucked into every tool box, tackle box and glove compartment simply because I can buy batteries in any convenience store or gas station almost anywhere in the world. They’re perhaps not my first choice in lights, but they are definitely my #1 fallback position.
I am very eager to get my hands on the new Thrunite offering once neutral versions become available and the teething problems are resolved. The last I heard, the release date has been bumped forward to Jan. 15. Admittedly, that’s about the fourth or fifth ‘bump’, so who knows.
I don’t think 2xAA lights are obsolete, if only for getting energy nearly everywhere and long runtimes.
Good choices are imho the Niteye Eye 12 and the Crelant V21AN.
Although the magnetic control ring in the Niteye is sweet i prefer the Crelant for the much better tint.
The thrunite neutron is a phenomenal light. I will wait for a deal on a NW to get one.
My take is that most sales of 2AA lights are to non-flashaholics. High current drivers require high performance cells - alkaleaks won't cut it.
While such drivers would do fine on Eneloops or other high grade NiMH cells, the general public still hasn't caught on to this in a big way. Manufacturers probably don't want the hassle of trying to explain why their "super light 2AA" only gives a few minutes of high brightness before the cheap batteries give out.
Simpler to just drop the output in favour of all round battery compatibility and useful runtime.
The Thrunite S15, Neutron and Armytek offerings are a great start but I think they will continue to sell only to a relatively niche market.
What would be nice is a few decent AA P60 hosts with high output drivers allowing us to build whatever we wanted.
One of my favorite and most used “small” lights currently is a modified 2-AA Olight S15.
Every time I see a 2AA light I see an Atreides I want to kill I see a light I want to shorten
My EDC is a eagletac D25A2 with a Nichia 219B high CRI in it…
I think because they are thin it makes it look longer than they are. I run my S15 in 2AA format as it’s rubbish on 1AA and lasts 5 mins on a 14500. Makes for a good torch in 2AA format. Sadly the tailcap magnet isn’t strong enough for the length when used like this though and it still f’ing activates in your pocket!!!
1x AA fits in a pocket, output seems limited by heat dissipation, so I have no interest in a 2X AA, jump straight to a 1x 18650.
In cheap lights, the driver seems the issue, 2x AA in Alkaline or Nihm isn’t going to direct drive typical high output emitters to close to the output of a Lion or more expensive driver. 3x AA isn’t bad, but well down the road of awkward sized.
Maybe the space is too narrow for the inductor of a high current boost driver. 3 x AAA is less battery, but it can do direct drive reasonably well. And being fat, it makes more room for the driver. I wish there were more 3 x AA lights to give to non-flashaholics. Added: and especially for children.
Personally I like the form of my Klarus P2A (2xAA) where as I do not like single cell versions (1xAA or AAA) as they are too small. I have big hands. But I also think that an 18650 torch is about as perfect as you can get size wise and of course capacity. But would always have a 2xAA or two in my armoury.
Marc.
That’s exactly my position as well. Ergonomics (U.I. & handling) is the top priority. Quality of construction and tint come close.
Then again, I’m mainly into small EDC lights. Those looking for workhorse beasts would probably care much more for brightness and endurance.
I think there is no such advantage and they are obsolete in terms of available technology about as much as incandescents are. That does not imply that 2*AA is not a good format for a mass market product. It's certainly one of the best as the masses are stuck with useless alkaleaks and sometimes "rechargeables" aka NiMH. I have personally never owned a single AA light, have previously had several 2*AA lights, and currently have no AA lights. The 2*AA size is the "normal flashlight" the flashlight-ignorant person is aware of. It's the "medium size" between keychain lights and huge ineffective C or D size Ma... incan flashlights.
IMO the single AA(preferably 14500) makes sense. So does the single AAA(or 10440) or single 18350 or 18650. But the double AA(or double 14500) most of the time has no form factor benefit at all. I't just something that's bigger than 1*18650 and as useful as single AA. Like a tiny Fiat with a leaky carb in the size of a school bus. I know even some flashaholics do want especially that form: the tube can be very thin on a slightly larger light. Some might even prefer 2*AAA. Just not my choice. Then again, I don't like putting flashlights in my mouth. 
Most of the time such lights have some aggressive tacticool anyway that would be bad for teeth.
Most of the time I really hate the AA hosts. They should be the minimum thickness all the way to make them as compact and pocketable as possible. The bulges, huge heads, sharp forms etc just ruin the product's usability. If one needs or wants a larger bezel or reflector, why not couple that to a real 18650 that makes the light effectively the same size? In a 140mm length, a 30mm head with 18mm tube is not any more compact than 30mm and 22mm, in my opinion. I prefer the simple tube form in small flashlights and that seems opposite to the typical 2*AA style. IMO, small lights should not be just miniatures of larger ones with long multi-cell tubes. They should have an entirely different design.
Or more simply, when 2*AA won't beat 1*18650 by a clear margin, it's obsolete. 
One of the first 2 x AA lights I modded was/is a Duracell plastic host with a curved green rectangular body and plastic reflector with the cells in a side by side 2S configuration. My guess is that this shape though easily molded is not as easily machined and is why in line tubes are more prevalent. It does fit my hand better than a AA Minimag though and has a side switch. For some reason I don’t like the asymmetry of the similar Dorcy light. Too ergo centric I guess.
My favourite 2-AA is the Olight S15 with the tube-extension (for 2-AA) its great output and excellent run times.
I have a MT2A too which sits in a clip on the pillar of my 4x4. Being nice and thin you hardly notice its there. Like anything, they have their place.
I think the wording of the thread is wrong, there is no way that double AA is obsolete, or on it’s way out, or outdated, or any such thing.
The thread should have been worded more about preferences, or opinions, I went through a stage where I would look at my Fenix LD22 and think that 18650s lights made it kind of fade away, but after being around 18650s more, I am glad that I own the LD22. and my LD12, and other AA and AAA lights.
The 2 AA is a wonderful size, and offers a little different performance than my single AA, it has it’s place, and for friends and relatives that are not flashlight people, it really shines over a 18650 light.
I own, and use, around 25 lights; and the 2AA lights that I have are the least used ones.
The reason is that if I wanted something really compact, then a single AA or 14500 would suffice.
But to carry something longer/bigger, a single 18650 with its longer run-time and massively greater output, negates the use of my 2AA lights every time. They (the single 18650 lights) are just so much more versatile. And only slightly heavier.