The latest update for Stadia has added an interesting new feature: tandem mode. This allows users to plug a secondary controller into the USB-C port on the Stadia controller, with the input working for both at the same time. Don’t be too excited though: the feature is meant to be easier to access and not an easy way to add multiplayer.
Other Stadia controllers and controllers from Xbox One and PS4 will work too (provided you have the correct cable or adapter). This may be a great way to help extremely young children play. Multiple Stadia controllers can use tandem mode at the same time as long as they are wirelessly connected to the Stadia.
Google is very confident that this is an experimental feature, so it may change or go away. Stadia’s features are a bit fluid at the moment, but it does reveal some of the unexpected things the Wi-Fi-based controller can do.
Source: Google via 9to5Google