Yes, the Line Input can be used for connecting external gear and to monitor stereo audio signals in real-time. The audio signal is even passed through the built-in EQ, Limiter, and Stereo Widener, Delay, and Reverb to create a well-balanced polished sound on Tracker's output.
Although the line input passthrough allows live stereo monitoring, yet when you are recording audio from the line input it will automatically get converted to a mono audio file. The audio input is offering an L/R mono choice for recording separate audio channels in the Config menu too.