According to a tweet from @TheSurfaceGuide (via Windows Central,), Microsoft's Surface Duo just got a small usability update. The custom version of the in-built on-screen SwiftKey keyboard now can split across both of the device's screens.
— The Surface Guide (@TheSurfaceGuide) November 6, 2020
Though the feature currently only works in Book Mode (where the device is held with both screens facing horizontally) this update helps make it easier to type on the device with both hands, and your thumbs. It might look like it is not available for you initially, but it is being reported that the app you're currently using will need to be spanned first before the spanning feature becomes active.
This is yet another usability improvement for Microsoft's dual-screen Android phone, which many initial reviewers criticized for bugs in the software. The company last updated the Duo back in October with a firmware update, patching the call stability and experience, touch stability, as well as overall device stability.