We often talk about solo artists and bands/groups with three members or more. What about the duo ? Two artists that are or have constantly recorded and performed together ! ...not a one shot performance of recording...like Elton John & Leon Russel !
The obvious one for me is Simon & Garfunkel...they're infamous.
Some others are Richard & Linda Thompson, Faith & Muse(goth),White Stripes, She and Him (thanks MH), Seals & Croft, The Finn Brothers (thanks CH) and Brooks & Dunn.
I'll stop here as I know the members have lots more to add !