Alternate, or take turns. This phrase is used either for two individuals alternately performing a task, as in Lifting cement blocks is such heavy work that they decided to change off periodically, or for one person alternately performing two tasks, as in I can concentrate on this book only for short periods so I frequently change off and work in the garden. Also see change of pace.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.