Collective motion of individuals among a group is a phenomenon observed in nature at all scales, from simple cells to fish, birds and even mammals. When bacteria swarm, their collective motion is typically characterized by densely-packed groups of individuals moving in coherent patterns of swirls and flows that can persist for several seconds. The swirling dynamics is a physical consequence of cell aspect ratio and cells density due to short-range steric effects and long-range hydrodynamic interactions. Thus, swarming bacteria are an intricate example of self-propelled particles and active matter. Previous studies focused on the average speed and density fluctuations of single-strain swarming colonies, where all cells in a single colony are identical. It was found that wild-type cells move differently from elongated mutants, having different modes of motions that also depend on the cell density. Here, in order to mimic realistic, in-situ, situations, where more than one species, or strain may inhabit the same niche, we mix two Bacillus subtilis strains that significantly differ in their aspect ratios and study their joint dynamics for a variety of cell densities. To distinguish between the strains, we use fluorescently labelled variants, in two colors, and a splitting optical system, to allow simultaneous dual tracking. The results indicated that the two strains are homogenously mixed, with dynamics that strongly depends on the partial ratio of each of the strains in the population. In particular, in wild-type-rich populations, the speed of each of the strains was not affected, compared to single-strain populations, but in long-rich populations, the speed of the wild-type decreased significantly, indicating that the long cells dictate the collective dynamics. In addition, the density fluctuations of each of the strains increased with increasing the partial ratio of each of the strains. These results indicate a new phase of collective dynamics compared to the recent phase diagram obtained for single-strain swarms.