Why were ther two actors playing Darren Stevens?

Answer 1

Because the original actor, Dick York, had a severe back condition due to an accident he had in 1959. This condition caused script rewrites and shooting delays. In January 1969 he collapsed on the set and was rushed to the hospital, so he left the show. That same month actor Dick Sargent took the role.

Answer 2

Dick York and then Dick Sargent played Darren Stevens on Bewitched.
Originally Dick York originally played Darrin and later the role was played by Dick Sargent in Bewitched.

Answer 3

because one almost whaned to quit