I think both artists are quite different when talking about how one reacts to their art; McCarthy's books make one experience the cosmic-awe of nature and ponder the inexplicable tragedies and ways of man. While Beefheart serves as the ying to this yang and seems to challenge this notion of determinism. He courts the shadow of the laws of nature, chaos.
However, these two artists are quite similar. Both are intensely reclusive and really give interviews if at all, both are extremely inspired by the desert, and their art owes much to the past as it is a part of it. They recycle-reuse-and-reinvent the past and return it to the cannon as if it was there all along. They borrow from the rich history of American folk culture; oral story telling, mythology, tale tales, folk, and blues. They take this ancient wisdom and shine its relevance, its timelessness.
I posted a couple of videos. The first is a short autobiographical documentary on Beefheart that sees him briefly talking about Miles Davis and has David Lynch pitching him a few questions.
The second series of videos is two excellent lectures on McCarthy's Blood Meridian by Professor Amy Hungerford from Yale University. I understand it is part of a series of courses from Yale that are available free online, check it out here.
McCarthy's Blood Meridian is widely regarded as one of best pieces of literature to come out in the last century. A heads up to people wanting to check out the lectures, it definitely helps if you have read the book but also if you were an english major it might remind you too much of school. Anyways the lecture is very well put together and sure to get you thinking. It sort of made me wish I had an english degree.