Boundaries of amplituhedra and NMHV symbol alphabets at two loops

In this sequel to [3] we classify the boundaries of amplituhedra relevant for determining the branch points of general two-loop amplitudes in planar <em>N</em> = 4 super-Yang-Mills theory. We explain the connection to on-shell diagrams, which serves as a useful cross-check. Here we determine the branch points of all two-loop NMHV amplitudes by solving the Landau equations for the relevant configurations and are led thereby to a conjecture for the symbol alphabets of all such amplitudes.