How to triangulate both ways? (2.80) I want to triangulate all my faces (quads) in a way that it makes an X. Effectively making four triangles for each face. How would I do that without having to manually do it for each face?

Unlike "Poke Faces", it should not create a vertex in the intersection.
Select a square grid of faces.  Make sure that you have a face as an active selection.  Use the checker deselect tool (from search menu) to select a checker of these faces.  Triangulate and set "Fixed" as the method in the operator box.  Select inverse and triangulate.  Set "Fixed Alternate" as the method in the operator box.