The 600 series headlamps only take 1 x 18650 up to 69 mm long, i recently bought a h600FW Mk II, eagletac 3400mah or 3100mah 18650 work fine as they are the shortest protected batteries avaliable, any longer and the tailcap won't make electrical contact with the body,
operating voltage is 2.7 - 4.2v, 2 x rcr123 will be around 5.4v, using 2 rcr123 batteries in a light are not the best idea anyway as they are low capacity only around 750mah so you will get less than half the runtime as 1 x 3400mah 18650,
The H602 is the most floody of the zebralight headlamps, it will give you a smooth 120 degree wall of light in front of you with no hotspot in the center, no throw as well as it has no reflector and the led is closer to the lens than the reflector models,
The H600Fw Mk II has the the same reflector as the standard H600 but with a frosted lens which smoothes out the hotspot in the center, so the H600FW is somewhere in between the H602 and the H600, gives a good flood with a nice diffused hotspot with a bit more throw than the H602,
i find the spot a bit distracting in headlamps with the reflector and clear lens, i like the H600Fw the best with the frosted lens and smooth hotspot, this is the first neutral /warm tinted light i have bought and i like it, gives a
warmer more natural light than the cool white tints,
I think the slightly higher output of the H602w compared to the H600Fw is due to the H600Fw having a frosted lens, this may block a little light, they measure what is coming out of the front of the light, OTF lumens,
Neutral / warm tinted leds have a slightly lower output than cool white leds too.