do double duty

1. To do two things simultaneously. It's sometimes tough to do double duty as an accountant and a mom, but we really needed a second income. I'm afraid you'll have to choose between playing football or being in the school band. You can't do double duty because both activities require too much extracurricular time. Because our receptionist is on vacation this week, I have to do double duty as fill-in receptionist and administrative assistant to the department.

2. To be able to serve two functions. Don't worry, this space can do double duty as a guest room and an office. The dining room table does double duty as the kids' homework space and the family dinner space. I just don't know that we'll have enough visitors to justify getting a more expensive couch that does double duty as a bed.