How can one have both astigmatism and far-sightedness?

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.
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.