I don't think the next Nintendo handheld will have dual screens. It's a redundant cost and often the bottom screen is just a meaningless battery suck to display a map or something.
The 2DS really being a single screen illustrates that even Nintendo is backing out of that. It was an idea that made sense 10 years ago because one large touch panel back then would've been incredibly cost prohibitive. Not today.
I don't think DS will be any part of the next Nintendo handheld branding. The next one is going to be designed specifically to integrate with a new Nintendo ecosystem and also be Nintendo's "answer" to the rise of tablet/smartphone gaming (DS/3DS is the past). It will be something entirely new and more in line with what consumers want today (no one uses the 2 screen solution Nintendo does -- for good reason ... there's little benefit to it).
DS and Wii as brands had great runs, but all good things come to an end and the world has changed so much in the last 8-10 years, Nintendo can't rest on outdated hardware legacies anymore, unless they want to continue year after year losing money.
