double as (someone or something)
To serve two separate purposes. Because we're understaffed, I have to double as receptionist and file clerk right now. You wouldn't know from looking at it, but this sleeping bag actually doubles as a poncho.
double as someone or something
[for someone] to serve in two capacities. The chairman will have to double as CEO until we find a new one. This table doubles as a desk during busy times.
To serve in some additional capacity: The high school principal doubles as the soccer coach.