Steve Lawrence Minus Gormé
"Pretty Blue Eyes" - 1959
Tony Orlando Before "Dawn" ( I say this somewhat tongue in cheek !)
"Bless You" - 1961
"Halfway To Paradise" -1961
This should be easier, but I am stumped. Most acts break up and then individuals go solo. I think the later sixties, seventies and eighties would be easier, because of the phenomenon known as the "super group".
The following is from the wikipedia article "super group":
"Some examples of the most well-known supergroups include: 1960s groups Cream, Blind Faith, and Crosby, Stills, Nash & Young; 1970s groups Emerson, Lake & Palmer, Humble Pie, Cactus, Toto and Bad Company; the 1980s & 1990s groups Band Aid, USA for Africa, The Firm, Traveling Wilburys, Asia, Damn Yankees, Power Station, and The Highwaymen..."