I see a lot of multiple 18650 lights but I also see a lot of people advising against running multiple LCRs. Would three 10440s in a solarforce L2i with a high voltage dropin be ok or a complete disaster? How about two 14500s in a 2AA light with a driver that can handle the voltage?
With a well chosen drop in you should be perfectly fine, IF the cells are as identical as possible. This means, they need to be the same capacity, cell chemistry and charging state. Otherwise one cell might charge the other when pressing the switch of the flashlight, and the resulting currents may cause the cells to vent or burst.
I used to drive a 4-18V XP-G drop in from solarforce in my L2 with a second battery tube and two 18650 2400 mAh flamed Trustfire cells. The cells were from one 2-pack, so I suppose the were from the same batch. They were always charged at the same time using a hobby charger, at around 0.7 C. I only used these cells in this flashlight. Despite this, after six months or so they had different resting voltages coming off the charger or out of the light. They did differ about 0.03V (charged). This does not sound much, but should amount to about 10-15% difference in capacity. So I decided to go back to using single cell lights only. I know I'm being a coward, but these cells are potentially hazardous, and I like to play it safe.
Many people do this with no ill effect, but it's not for me. Just check your cells with a DMM, and keep an eye on their behaviour, and you should be fine.
I'm doing just that with my L2r. So far I like it but we'll see. My ramblings about it are in the AA sub-forum.
timelyFoy
I have no scientific background on batteries but I think that two cells in series is not big deal if they are as similar as possible (same times charged, baught as pair when possible, charged by the same charger and voltage checked every time before used in series). Good quality protected batteries is of course a huge plus.
Three+ lithiums in series... Not for me. Too much energy and many things that can go wrong... Maybe two good batteries pushing 8.4V / 3A to a not so good battery... 
Again, no scientific background on batteries, this is what I understand from what I read from people much more educated on batteries than me. I might be totally wrong.
I have two flashlights running on two 18650 each. I use new batteries of the same pack in each of them. I mesure the voltage before and after the recharge. As soon there is a significant difference in voltage (normally they are within 0,03 V) I replace them with new ones and use the older batteries in other lights that need only one of them. I allready thought of drilling a hole in the side of the body tube and put a plastic stopper (some kind of a safety venting hole), but I didnt do it yet.
Hm... this would help against slow venting, but in the case of a full on explosion a hole might be to small, and the battery tube will still burst. And to be honest, you do monitor your cells, so damaging the looks of your lights by drilling into them seems unnecessary.
The only cases of explosions I read of occured when someone did something really stupid, like mixing full and empty cells.
About all I can add is that it would be wise to check the voltage of the cells often until you become familar with their performance in your light. Having a good feel for the runtime of the cells in your light will let you avoid too deep a discharge. In other words, don't depend on the cells' protection circuits to tell you when to recharge.
When I feel like it would be time for recharging, the cells are normally still well above 3,5 V. It never happened to me to see a cell cutting off.
run a good 18650 or run imr
sorry I'm stuck on why /
people put their fingers in fan blades too ..why ?? cuz they were curious ..
Remember one silly thing the moment you ramp up a light and it emmittes lots of lumens ..Your EYE probably smarter than your brain shuts it's self right down ..
Is it true that using two li ion cells as the tk35 does is safer ? Or am I out to lunch here? I am a newb as well but this is good discussion:)
Well done then.
Some of us are not as careful as you, and may not recharge often enough.
What works for me is:
If you have used it, charge it.
OK if you want the maximum life from your cells don't do this, but how much do 18650s cost?
And how much do nice lights cost?