  X-ray emission provides the most direct diagnostics of the energy-release
process in solar flares. Occasionally, a superhot X-ray source is found to be
above hot flare loops of ~10 MK temperature. While the origin of the superhot
plasma is still elusive, it has conjured up an intriguing image of in-situ
plasma heating near the reconnection site high above the flare loops, in
contrast to the conventional picture of chromospheric evaporation. Here we
investigate an extremely long-duration solar flare, in which EUV images show
two distinct flare loop systems that appear successively along a Gamma-shaped
polarity inversion line (PIL). When both flare loop systems are present, the
HXR spectrum is found to be well fitted by combining a hot component (Te ~12
MK) and a superhot component (Te ~30 MK). Associated with a fast CME, the
superhot X-ray source is located at top of the flare arcade that appears
earlier, straddling and extending along the long "arm" of the Gamma-shaped PIL.
Associated with a slow CME, the hot X-ray source is located at the top of the
flare arcade that appears later and sits astride the short "arm" of the
Gamma-shaped PIL. Aided by observations from a different viewing angle, we are
able to verify that the superhot X-ray source is above the hot one in
projection, but the two sources belong to different flare loop systems. Thus,
this case study provides a stereoscopic observation explaining the co-existence
of superhot and hot X-ray emitting plasmas in solar flares.
