Using Lithium-ion batteries in series
If you put two unprotected cells together by yourself, be sure to balance them first. So you don't risk overdischarging or overcharging one of the cells. Charging a fully depleted Lithium cell can get you in trouble...

Don't know about DS cells but they don't seem to have protection circuits:
Check this out, he added a protection circuit:
http://www.instructables.com/id/Solar-DS-quotLightquot-Redone-and-Greatly-Impr/

Regarding cell phone batteries, I know that at least one large manufacturer used to have most of their battery protection circuits in the phone, not the battery. So be careful. 

Basically you need to protect your cells from: over voltage, under voltage, over current and very high temperatures (and maybe charging during very low temp).