It is possible to Slant, Shear or use Perspective on an image vertically or horizontally but not both ways at once. The direction you move the handle first is the only direction you will be able to shear unless you restart the process. The simplest method for shearing both directions on one image is:
1. Select the image you want to shear
2. Choose Shear from the Transform menu
3. Drag one of the handles up or down
4. Now try to drag any handle left or right (notice that this is not possible)
5. In order to shear horizontally, click once outside the image (notice that the image is still selected but that the handles are gone, clicking twice deselects the image)
6. Choose Shear from the Transform menu again
7. Now you can drag left or right
8. Repeat steps #5 - 7 in order to switch shearing directions again
There is no keyboard equivalent to change shear directions.