William Ashbless was invented by Powers and Blaylock when they were in school; they quoted extracts from Ashbless' poems in their English essays, and reportedly fooled their teacher into believing him to be real. Ashbless also appears in a couple of Blaylock's novels.
Powers and Blaylock refer to each other in their novels; there is a "Captain Powers" in one of Blaylock's books (Homunculus), and in one of Powers' there is a ship named the "Blaylock".
Recently (2001), Ashbless "authored" the book On Pirates with an introduction by Powers and illustrations by Blaylock! [ISBN 1931081220 (amazon.com, search)]