  In nature, bacterial collectives typically consist of multiple species, which
are interacting both biochemically and physically. Nonetheless, past studies on
the physical properties of swarming bacteria were focused on axenic (single
species) populations. In bacterial swarming, intricate interactions between the
individuals lead to clusters, rapid jets and vortices that depend on cell
characteristics such as speed and length. In this work, we show the first
results of rapidly swarming mixed-species populations of {\sl Bacillus
subtilis} and {\sl Serratia marcescens}, two model swarm species that are known
to swarm well in axenic situations. In mixed liquid cultures, both species have
higher reproduction rates. We show that the mixed population swarms together
well and that the fraction between the species determines all dynamical scales
- from the microscopic (e.g. speed distribution), mesoscopic (vortex size) and
macroscopic (colony size structure). Understanding mixed-species swarms is
essential for a comprehensive understanding of the bacterial swarming
phenomenon and its biological and evolutionary implications.
