The Nexus 6P can charge the Nexus 5X via USB Type-C
Both the Nexus 6P and 5X come with USB Type-C ports and USB Type-C cables for charging. Interestingly, it's been discovered that, if you connect the Nexus 6P to the Nexus 5X via a USB Type-C cable, the former starts charging the latter. Thus, the large 6P (which has a 3450 mAh non-removable battery) can act as a power bank for the smaller 5X (which comes with a 2700 mAh battery, also non-removable).
According to our tests, both the Nexus 6P and Nexus 5X offer average battery life (see here and here). The 6P isn't better than the 5X in this regard because, although it has a larger battery, it features a 5.7-inch Quad HD (1440 x 2560 pixels) screen, while the 5X only offers a 5.2-inch 1080p display.
For a closer look at Google's latest smartphones, check out our Nexus 6P unboxing and our Nexus 5X unboxing, and stay tuned for our reviews of both devices.