The upcoming major Windows overhaul codenamed “Threshold” (Revealed first by a Microsoft employee on Reddit) will finally bring a single Windows SKU to run on both Phone and Tablets. It will also bring the ability to multitask apps in side by side windows in a single view. As per ZDNet’s Mary Jo foley,
The combined Phone/Tablet SKU of Threshold won’t have a Desktop environment at all, but still will support apps running side by side, my sources are reconfirming. This “Threshold Mobile” SKU will work on ARM-based Windows Phones (not just Lumias), ARM-based Windows tablets and, I believe, Intel-Atom-based tablets.
She also claims that Microsoft is expected to deliver a public preview of the Threshold release, most likely in the fall of 2014. That looks bit in line with what our sources predicted for Windows Phone 9 preview. According to Mary it may or mayn’t be called Windows 9 ultimately.