  We take advantage of the deep and wide coverage of the VST ATLAS survey to
study the line-of-sight structure of the Sagittarius stellar stream in the
Southern hemisphere, only ~40{\deg} away from the progenitor. We use
photometrically selected Sub-Giant Branch (SGB) stars to reveal a complex
debris morphology of the trailing arm and detect at least two clear peaks in
the SGB distance modulus distribution. The separation between the two
line-of-sight components is at least 5 kpc at the edge of the VST ATLAS
footprint, but appears to change along the stream, which allows us to conclude
that these detections correspond to two physically independent stellar
structures, rather than a mix of co-distant stellar populations within a single
stream. Our discovery of a fork in the Sgr trailing arm is verified using Blue
Horizontal Branch stars and our distance measurements are calibrated using RR
Lyrae stars from the Catalina Real-Time Transient Survey. Comparing with
numerical simulations of the Sgr dwarf disruption, the more distant of the two
components in the fork matches perfectly with the track of the trailing debris.
However, no obvious counterpart exists in the simulation for the closer
line-of-sight component.
