Third value "twodimensional" for aria-orientation

[MarioBatusic]

Description of bug or feature request

We intend to use the new, interactive lists also for dashboards and other 2-dimensional displays. This concerns non-grid multi-line arrangements such as Home on Windows, cards display in the File Explorer, etc.
In grids - ttables it is clear even for screen reader users that navigation is possible both horizontally and vertically. In lists, on the other hand, the list orientation - i.e. the possible arrow navigation - must be specified using the aria-orientation property.

This property has currently only two valid values: "horizontal" and "vertical".

We need a third value: "both" / "two-dimensional" or similar to correctly specify the dashboard-like arrangement of the list entries for screen readers.
For me there is one ambiguity that could pose a real problem: Do the existing AAPIs allow the orientation to be extended with an additional value? Or could the screen readers interpret the default value of "undefined" as "two-dimensional"?

Will this require a change to CORE-AAM?

Will this require a change to the ARIA authoring guide?