The documentation explains it kind of poorly, but the idea is that you add an axis mapping for each axis in the UE4 Input menu, using identical names and the _X and _Y suffixes. This makes the SteamVR plugin combine them into a single SteamVR action.
As an example, this is a setup for two 2-dimensional movement actions, one for each thumbstick. The MovementLeft_X and MovementLeft_Y combine into a single MovementLeft action for the left controller, and the same with MovementRight for the right one.
