For the past decade or so, the French-born Laceque sisters have single-handedly transformed the rather prissy art of duo piano playing. Not overly flamboyant, they tend to approach a piece of music with the grace and flair of a pair of synchronized swimmers. Both are riveting presences onstage; Katia is more reserved and elegant, while Marielle, who jams with John McLaughlin's jazz band, is more vivacious and spontaneous. The sisters have overcome the limitations of the duo-piano repertoire by rearranging existing works and commissioning new ones, and their range now encompasses both Baroque and contemporary pieces. A longtime favorite of local recital-going crwods, the sister act makes it Ravinia and orchestral debut at this Chicago Symphony Orchestra show. For the occasion the Labeques have chosen the Concerto for Two Pianos and Orchestra by Bohuslav Martinu (1890-1959), the most important Czech composer of this century. This 1943 composition was written shortly after the composer, unable to return to his homeland from France because of the Nazi invasion, fled to the U.S. The pianos are the stars of the piece, their percussive sonorities exploited to the hilt in the expression of sorrow and anger. This all-Czech affair also marks the CSO debut of Libor Pesek, a fine Czech-born maestro who heads the Royal Liverpool Philharmonic. (On Saturday, Pesek will conduct pianist Peter Serkin in Stravinsky's Concerto for Piano and Wind Instruments--another noteworthy event). The rest of the program consists of the overture to Janacek's psycho-opera From the House of the Dead; Dvorak's seldom-performed Symphony No. 7; and, alas, the inevitable "Moldau" from Smetana's patriotic ode Ma Vlast. Tonight at 8; pavilion, Ravinia Park, Lake Cook and Green Bay Rds, Highland Park; 728-4642.