Rolling out now to Nexus and Pixel handsets running on Android 7.1.1 is a new feature called Instant Tethering. With this new application, anytime one of your registered Google devices loses a cellular connection, it will automatically send a request to allow the device to tether with your other Android phone or tablet. No setup is required, thanks to Bluetooth. The screen will also show you the battery life of the other device.
Instant Tethering will be available first for Nexus and Pixel devices running on Android 7.1.1. The Nexus 9 and Pixel C tablets cannot send out a Wi-Fi signal and thus will only connect to your compatible phone. This will work on Marshmallow as well as Nougat. Meanwhile, two phones will be able to work with each other if one of their signals goes down.
The Nexus and Pixel devices will be receiving the feature first. To see if it has arrived, go to Settings > Google and if you see a listing for the new feature, it means you now have it on your device. If it doesn't show up yet, be patient. Instant Tethering is just now starting its long and winding road through Android phones and tablets.