Prolific conductor and composer Michael Tilson Thomas is so in demand he often needs to appear in two places at once. On any given Sunday he can be found at the Lincoln Theatre in South Beach conducting an afternoon performance by the New World Symphony, where he's artistic director. Just a bit later, there he is on the other coast, wielding the baton at a San Francisco Symphony concert broadcast at 9:00 p.m. on WTMI-FM (93.1). What's even more amazing than Thomas's apparent faster-than-the-speed-of-sound feat is the amount of advertising during this two-hour broadcast: Commercials run only between works. An unusual approach for revenue-hungry WTMI, which has resorted to playing more movie soundtracks and hackneyed orchestral versions of hackneyed show tunes in an effort to widen listenership. Apparently on Sunday nights some things are still sacred.