@idiggplants, that only works in parallel, and only with small differences.

In series, it will not make cells explode obviously, but will drain one 18650 faster than the other, and can lead to problems down to the line.

IMO you should just go with an Emisar D1S, and a spare 18650.

IMO, I think you should

Always charge series cells together, with similar capacity and internal resistance.