Some upright MagSafe charger designs don't work well with the iPhone Duo when it's unfolded. As demonstrated by Mrwhosetheboss (Arun Maini), the uneven weight of the iPhone Duo can cause it to tilt on a charger when it's open.
The MagSafe ring on the back of the iPhone Duo covers only one half of the device, and some chargers don't have strong enough magnets to hold up the Duo with the extra weight on one side. iPhones do have an extra anti-rotation magnet at the bottom of the magnetic charging ring, but some chargers aren't designed to use it.
On Reddit, some users pointed out that *The Verge*'s Nilay Patel demonstrated the iPhone Duo charging horizontally on a MagSafe charger with no issue, so the problem may not be universal. Charging with the iPhone Duo open in portrait orientation likely works, but the safest way to use upright MagSafe stands appears to be with the iPhone Duo closed. A flat charger won't have the same rotation issue.
Companies will likely make wider stands with a ledge to hold the Duo upright when unfolded. Until then, anyone planning to use the Duo on a desk should expect positioning limitations with some MagSafe chargers. The iPhone Duo launches on Friday, October 23.
Source: MacRumors · Summarized by HeadlinesBriefing