The best practice to make a multicell battery (parallel or series) is to have two brand new identical batteries. Reason being they need to wear identically.

Ideally they also should be spot welded. Batteries, even bog standard NiMH (let alone li-ion) cells should never be soldered.

In regard to the dual battery. I understand your concern. I've done two. I find it quite stressful having a cell without any protection sitting there with me and a hot thing pointed at it. From memory, the case is positive and the smaller button is negative.