Chicken or not, I'm not intersted in seeing what happens when you power up a 2 x 18650 light with one cell significantly discharged relative to the other.
oranyotherlionmadnessFoy
Sensible. I've had cells die on me with no drama at all but have exactly zero interest in provoking it nowadays. That's what multimeters are for.
Booming Li Ion cells can do Bad Things. Really don't breathe the smoke - really, really don't breathe the smoke. Enough calcium gluconate in time might just save your lungs but don't bet on it - you are, after all, betting your ability to breathe. And I've been a smoker for the last 36 years.
Assume two cells of same brand and nominal capacity:
Is matching their voltage enough? Could they be borrowed from two single-cell flashlights that have seen a considerably different amount of use in the past?
For lithium ion cells, yes. Voltage correlates reasonably closely to retained charge. For cells of the same brand and capacity.
Outside that, check more carefully.
For primary (disposable) cells it might be different. I don't do primary cells.
I read you load and clear.
Not particularly afraid of dual 18650 serial config, but i'm all against triple in series. That is asking for trouble.
Is that just due to the higher probability for one rotten egg to happen, or are there other reasons that make a three-cell serial configuration even more questionable?
Variety of reasons:
1 cell - The light will get dim or the cell's protection will shut it down. Result: Little or no damage to the cell, no reverse polarity. Worst thing is you may shorten the life of the cell.
2 cell serial - You may notice the light get dimmer, but you may not, because the stronger cell will compensate for weaker one. If you don't notice and shut off the light, the stronger cell will keep providing power and possibly cause the weaker cell to reverse polarity. That is when it gets dangerous.
3 cell serial - Pretty much the same as #2 but worse because you're less likely to notice the light getting dimmer and the two stronger cells are more likely to cause reverse polarity in the weakest cell. More cells means higher chance of one being weaker than the others.
Cell protection might shut a cell down in time, but it might not. Cell protection circuits can and do fail (even with non-budget cells) and it's considered unwise to rely upon them. Many budget cells are not protected, and we're talking about 18650 here - they contain a lot energy.
Don't get me wrong - I'm not saying don't do it. Lots of people run multi-cell configurations and don't have problems. But they (hopefully) understand the risks, own and use a DMM, and know how to select and monitor the cells used for this purpose.
Very informative, photon1k, thank you.
Thank you for asking the question. It probably doesn't get asked often enough. And please don't just take my word for it. Do some reading about the care and feeding of lithium-ion cells. You may be glad you did.
Right. I did that before starting with 18650 and I treat these with due respect. (Although, after watching the videos from Don's link, I might want to find a more effective fireproof container than the baking pan I've been using for charging.) 
It was the reverse polarity prevention bit that I wasn't quite clear about.
very interesting reading here…but for now I have another problem…one of my 18650 battery didnt work with hi-mode of my KD C8 XML everytime the light shuted down and only work in low and mid modes,the second 18650 was OK but few days ago the same scenario occured and I figgured out that when the batteries are a little bit discharged they work in high mode with no problem!!! so where is the problem? batteries are not good? driver is not good? did charger damage the batteries? please help ASAP as I will receive new batteries this week and dont want to damage them…many THX