  Theoretical models and numerical simulations have established a framework of
galaxy evolution in which galaxies merge and create dual supermassive black
holes (with separations of one to ten kiloparsecs), which eventually sink into
the centre of the merger remnant, emit gravitational waves and coalesce. The
merger also triggers star formation and supermassive black hole growth, and gas
outflows regulate the stellar content. Although this theoretical picture is
supported by recent observations of starburst-driven and supermassive black
hole-driven outflows, it remains unclear how these outflows interact with the
interstellar medium. Furthermore, the relative contributions of star formation
and black hole activity to galactic feedback remain unknown. Here we report
observations of dual outflows in the central region of the prototypical merger
NGC 6240. We find a black-hole-driven outflow of [O III] to the northeast and a
starburst-driven outflow of H{\alpha} to the northwest. The orientations and
positions of the outflows allow us to isolate them spatially and study their
properties independently. We estimate mass outflow rates of 10 and 75 solar
masses per year for the H{\alpha} bubble and the [O III] cone, respectively.
Their combined mass outflow is comparable to the star formation rate,
suggesting that negative feedback on star formation is occurring.
