More success as a duo than in solo careers helped keep things going, I think. Looking at the "Years Active" sidebar on Wikipedia helps illustrate that:
1956–1958, 1963–1970, 1972, 1975–1977, 1981–1984, 1990, 1993, 2003–2005, and 2007–2010
A great deal of amazing work has come out of stormy partnerships.