Watching the early Mickey Mouse cartoons made me realize that the majority of early characters were missing something: A partner. Felix, Betty Boop, and Popeye - to name a few - were all originally featured on their own.
I'm not sure when the first true "duo" emerged, but Tom and Jerry may have been the earliest to become superstars. Their first cartoon nearly won an Academy Award in 1941, setting the tone for duos like Heckel and Jeckel (1946), the Road Runner and Wile e. Coyote (1947), and eventually the amazingly derivative Courageous Cat and Minute Mouse (1960)
posted by s.st.f.d.