When two cells are in series and one of the two cell have a different state of charge, one cell can overheat causing heat buildup or gas comming out of the vent hole on the positive side of the cell. Since those cell are in an enclosed container in the body of the flashlight, this may lead to high pressure build up inside the body of the flashlight leading to an explosion of the housing. This is rare but has happen to some member at Candle power forum. Note: this effect is not limited to just 2cell 18650, but it also occurs in 2 cell CR123 batteries.

It is important to periodically check the final charge voltage of the cell after fully charge. There can be a chance where one cell was charged to 4.2v while the second cell did not charge at all. If both of these unbalace state of charge cell were to be use together in a 2cell 18650 flashlight, unpleasant result may occur.

Know how well your charger are working and learn how to use a digital volt meter and you should be alright.

BTW one important note: when choosing 18650 cell, stay with the cell that are protected. These cell has a small PC board at the negative side of the battery to protect things like over charging, over discharge and shorts. Most cell are sold as protected cell.