double as

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.
See also: double
Farlex Dictionary of Idioms.

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.
See also: double
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

double as

v.
To serve in some additional capacity: The high school principal doubles as the soccer coach.
See also: double
The American Heritage® Dictionary of Phrasal Verbs.
See also:
  • double as (someone or something)
  • be of service
  • be of service (to someone or something)
  • cross purpose
  • stand the sight of (someone or something)
  • bear the sight of (someone or something)
  • engage
  • engage (one) as (something)
  • engage as
  • be at (one's) command
References in classic literature
A well constructed plot should, therefore, be single in its issue, rather than double as some maintain.
Poetics of Aristotle
(In this sense, the fiction may double as a kind of blueprint, or user's manual, to a parallel world.)
Double trouble
9; or we double as though there is a high post, with the point defender (X2) becoming our trapper, as shown in Diag.
The post ambush
That finding flies in the face of some famous modern choices, like having Theseus and Hippolyta double as Oberon and Titania, as in Brook, 1970 (Alan Howard and Sara Kestelman) or Caird (John Carlisle and Claire Higgins), a doubling that violates, among other things, "the law of re-entry." King suggests that even minor actors who doubled many roles usually had a scene off-stage in which to change costumes and any "actor who doubles in principal parts is off-stage for an interval of at least one full scene between roles" (29).
Casting Shakespeare's Plays: London Actors and Their Roles