The Collaborative International Dictionary of English v.0.48:

doubleheader \doubleheader\ n.
   two events, usually entertainment or sports performances,
   occurring immediately in succession or within a short time of
   each other; especially, two sports games played by the same
   teams on the same day, one right after another.

   Note: A doubleheader in baseball is often scheduled between
         teams when a previously shceduled game is postponed, as
         due to rain or other interfering factors. In such
         cases, the admission price for the doubleheader remains
         that of a single game, though two games are played.

   Syn: twin bill, double feature.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

doubleheader
    n 1: two games instead of one (especially in baseball when the
         same two teams play two games on the same day) [syn: twin
         bill, doubleheader, double feature]