According to the first tests conducted by TechInsights, the dual battery mounted on the iPhone X provides about two hours more autonomy than the iPhone 7.
On the official website, Apple reports that the iPhone X battery is the same as for telephone conversation times, but less so for video playback and web browsing than iPhone 8 Plus and iPhone 7. The two-cell battery has a power of 10.35 Wh (2.716 mAh at 3.81 V), almost equal to the single cell of 3.82 V, 2.691 mAh and 10.28 Wh of the iphone 8 Plus.
According to TechInsight, it is the second battery that adds about another two hours of autonomy to the iPhone X, although it does not specify what kind of use it is.
In the coming days we will also be testing to understand the real autonomy of the iPhone X.