A few that immediately come to mind include:

Johnny Cash and June Carter Cash
Conway Twitty and Loretta Lynn
Porter Wagoner and Dolly Parton
Kenny Rogers and Dolly Parton
Homer & Jethro :lol:
The Judds
Brooks & Dunn

George Jones and Tammy Wynette
Waylon Jennings & Willie Nelson
Merle Haggard and Bonnie Owens
The Louvin Brothers
The Wilburn Brothers
The Bellamy Brothers
George Jones and Melba Montgomery
Merle Haggard and Willie Nelson
Waylon Jennings and Jessi Colter
Jim Ed Brown and Helen Cornelius

*the judds
*johnny and june carter cash
*montgomery gentry
*faith hill & tim mcgraw

My favs are:
The Judds
Motgomery Gentry
The Wreckers
Sugarland (They used to have a 3rd person with them but now it's just the two of them)
Waylon and Willie