The latest update to Stadia added an interesting new feature: Tandem Mode. This allows users to plug in a secondary controller to the USB-C port on the Stadia controller, with the input working for both at once. Don’t get too excited, though: the feature is intended to be an aid to accessibility, not an easy way to add multiplayer.
That said, it’s a cool idea if you have trouble using a standard game controller. Stadia’s support documentation explicitly mentions the Xbox Adaptive Controller, a gadget designed with incredibly variable input options. Pressing buttons on the secondary connected controller will mimic the same buttons on the Stadia controller.
Other Stadia controllers and controllers from the Xbox One and PS4 also work (as long as you have the correct cable or adapter), which might be a good way to help extremely young kids play. Multiple Stadia controllers can use Tandem Mode at the same time, so long as they’re connected to Stadia wirelessly.
Google is very clear that this is an experimental feature, so it may change up…or disappear entirely. Stadia’s features are a bit fluid at the moment, but it’s demonstrative of some of the unexpected things that the Wi-Fi-based controller can do.