Colin Andrews and Janette Fishell are a rarity: they're an organ duo. If you think piano duets look tricky, you'd be amazed by the amount of choreography--of hands and feet--required for two organists to share one console. Unlike its piano counter part, which historically caught the fancy of many a composer and of the rising middle class, the organ duet has been far less popular--for the obvious reason that few 19th-century families could afford a pipe organ. Purcell is believed to have dashed off an organ duet, but a piece from the 18th century by British composer Samuel Wesley is usually regarded as the first entry in this literature. Duet playing only became more extensive in this century, when mechanical innovations made the coordination less of a physical challenge. Still, organ duet has remained an estoric art, embraced mostly by highly skilled players who can get along with each other; surprisingly, almost all duos--professional and amateur--are husband-and-wife teams. Andrews and Fishell pursued distinquished solo careers before they married in 1989 and took their act on the road. The British-born Andrews has studied with Gillian Weir and often appears on the BBC, and Fishell, now a prof at East Carolina University, holds a doctorate in organ performance from Northwestern, where one of her mentors was Wolfgang Rubsam, the head organist at Rockefeller Chapel. (Both, by the way, continue to solo.) Tonight, the couple will perform on Rockefeller's Skinner, a symphonic organ well suited to four-hand playing. Given the understandable slimness of the repertoire, their program--divided into British and eastern European halves--holds a number of transcriptions. Of particular interest are: a section from Gustav Holst's The Planets, Mikhail Ippolitov-Ivanov's Caucasion Sketches, a Toccata and Fugue by Hubert Parry, and the local debut of two works by Petr Eben, a contemporary Czech composer and a Fishell favorite. Friday, 8 PM, Rockefeller Chapel, University of Chicago, 5850 S. Woodlawn; 702-2100.