  We report a study of a compound solar eruption that was associated with two
consecutively erupting magnetic structures and correspondingly two distinct
peaks, during impulsive phase, of an M-class flare (M8.5). Simultaneous
multi-viewpoint observations from $\textit{SDO}$, $\textit{GOES}$ and
$\textit{STEREO-A}$ show that this compound eruption originated from two
pre-existing sigmoidal magnetic structures lying along the same polarity
inversion line. Observations of the associated pre-existing filaments further
show that these magnetic structures are lying one on top of the other,
separated by 12 Mm in height, in a so-called "double-decker" configuration. The
high-lying magnetic structure became unstable and erupted first, appearing as
an expanding hot channel seen at extreme ultraviolet wavelengths. About 12
minutes later, the low-lying structure also started to erupt and moved at an
even faster speed compared to the high-lying one. As a result, the two erupting
structures interacted and merged with each other, appearing as a single coronal
mass ejection in the outer corona. We find that the double-decker configuration
is likely caused by the persistent shearing motion and flux cancellation along
the source active region's strong-gradient polarity inversion line. The
successive destabilization of these two separate but closely spaced magnetic
structures, possibly in the form of magnetic flux ropes, led to a compound
solar eruption. The study of the compound eruption provides a unique
opportunity to reveal the formation process, initiation, and evolution of
complex eruptive structures in solar active regions.
