double duty - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Fulfilling two functions at the same time, as in Our pickup truck does double duty, since it is used for the business and for family outings.