By the way, is music a richer source than print?
If two people or instruments perform together, it is called a duet.
If two people write a piece together, it has no special name. If two poems, books, essays, etc. go in the same series, it has no special name that I know of.1
Three in music gives one a trio, and in literature, a trilogy.
Lit has sequel and prequel, but no equivalent of twin or octogenarian, for that matter.
So, what's lit speak for a quartet?2
Series and saga do multiples, but not in specifically like quintet, sextet, octet, etc. and in prayers, novena provide.
1 But, in a book I just started reading, The Penwyth Curse by Catherine Coulter, the "Titles by Catherine Coulter" page lists her output by categories, such as The Bride Series, followed by a list of seven book titles, the first four of which end in Bride; The Star Series, four novels all ending in the word Star; The Legacy Trilogy, right, three ending in the expected word, etc. The grouping of interest to me is for Devil's Duology, a list of two with Devil in the title. Sounds good to me. (appended 8/22/08)
2And when I was checking names of poetic feet in Harry Shaw's 1972 edition of Concise Dictionary of Literary Terms, right above tetrameter, four feet per measure, I discovered tetralogy, four works that make up a set, a series of four related novels, operas, or plays... (appended 8/25/08)