Well yes but actually no. The phrase "dual screen" is descriptive yes. But that is not what Nintendo called their product. The official name for the Nintendo DS is, well, Nintendo DS. So they very well could have trademarked the "DS" naming to keep for themselves for all eternity. Especially since they could argue that the shortened "DS" is distinctive since it is an abbreviation and not just a plain description.
All I'm saying is I'm surprised the asshole suits at Nintendo didn't do it, don't know if they tried but failed though.
It would actually be trademark and not copyright, and as such it requires constant market use by the trademark holder, and Nintendo has stopped using the DS trademark