CART:
Emerson Fittipaldi, World Champion 1972 and 1974
Mario Andretti, World Champion 1978
Nigel Mansell, World Champion 1992
F1:
Alain Prost, World Champion,1985 1986 1989 1993
Ayrton Senna, World Champion 1988 1990 1991
A quick check shows that three former World Champions raced in the CART series in 1993 and two former world champions raced in the F1 Championship in 1993. I usually spend more than a few minutes researching but i'm on my way out the door for the rest of the day so don't shoot me if I missed someone.