Although the two ensembles have fought heartily for the attention of the surprisingly large audience for contemporary recordings of medieval vocal music, the ladies of Anonymous 4 and the six gentlemen of Lionheart join their polyphonic forces tonight in a presentation of sacred music by medieval Flemish composer Johannes Ockeghem. Anonymous 4 has held its own against the gents for quite some time and is the reigning champion of plainchant, with several albums under its belt, including the top-selling A Lammas Ladymass. Lionheart, however, is riding on the success of last year’s Paris 1200, a collection of interpretations of 12th-century works by Masters Leonin and Perotin. WETA’s Robert Aubry Davis moderates a discussion two hours before the concert, which includes Ockeghem’s masterwork Missa de Plus en Plus and begins at 8 p.m. at the Washington National Cathedral, Wisconsin & Massachusetts Aves. NW. $15-25. (301) 405-7847. (Amy Domingues)