How can one have both astigmatism and far-sightedness?

Different things: Farsightedness is caused by the globe of your eye too short to focus the image on the retina. Astigmatism is caused by your cornea being off-round on one or more planes.

Architecture: Think of the eye as a sphere; everyone has a slightly different size sphere - some longer and some shorter. Farsighted individuals have shorter eyeballs and cannot get far enough away from objects to see them clearly. Astigmatism describes how "un-round" the front window (cornea) is. The more oblong the cornea, the more distorted images are, regardless of whether you are farsighted or nearsighted.