Fenix rates their lights in 'out the front' (OTF) lumens, ie they are measured with a lumenometer (or whatever it's called).
You use an Integrating Sphere, sometimes called a Globe. Lumens is a measurement of the total level of illumination that a light source produces. In the case of an LED, this is generally spread over 120+ degrees. The integrating Sphere "collects" all the available light, and directs it to a single point at which it can be measured.
Lux is the measurement of the amount of light at a single spot. This figure is usually fudged by Manufacturers with lesser scruples by measuring it at a very short distance, as the Light output falls with an increase in distance to the point measured.
OTF lumens are measured in a very similar way to Lux, rather then using an Integrated Sphere.
With regards two cells vs one, the XM-L T6 has it's maximum output at (Off the top of my head), at 3.35v. Given that you can discharge a Li-Ion/LiPo battery to 3.2v quite happily (with 2.5v being the bare minimum before it refuses to charge again), then 2 cells will increase the time you can run at the maximum rated output before falling voltage will drop the output.
Searching for the ever elusive Underground Titty Bar.
DDDWH CC