CLASSICAL Two discs: ''The Tango Project.'' William Schimmel, accordion; Michael Sahl, piano; Stan Kurtis, violin. (Nonesuch Digital D-79030.) Also, ''Domingo Sings Tangos.'' (DG 2536 416.) - Two records that tackle the same genre of music. The Nonesuch release is enchanting musically and beguiling sonically. The trio of instruments used captures just the right sound for the music; the players treat the music seriously yet lovingly, and each side bubbles and bounces with rare spontaneity and period authenticity. The Domingo record is, in its way, equally fine, though hardly authentic in sound. Instead, the famed tenor is backed by a Hollywood-studio symphony orchestra. He sings gloriously, avoiding the problematic top register altogether. He projects words with passion and sounds uncommonly relaxed and free-ringing. A richly pleasurable album that is a must for Domingo fans as well as anyone interest in the repertoire.