Twitty and Lynn — and their internationally popular show “A Salute to Conway & Loretta” — aren’t impersonators. Tre doesn’t groom Seventies sideburns and perm his hair; Tayla doesn’t mimic her grandmother’s mannerisms. Rather, they are onstage celebrating country music, the lost art of duet singing, and the two beloved icons they refer to as “Poppy” and “Memaw.”