Three dimensional dunes of crescentic shape, called barchans, are commonly found on Earth and other planetary environments. In the great majority of cases, barchans are organized in large fields in which corridors of size‐selected barchans are observed, and where barchan‐barchan interactions play an important role in size regulation. Previous studies shed light on the interactions between barchans by making use of monodisperse particles, but dunes in nature consist, however, of polydisperse grains. In this paper, we investigate the binary interactions of barchans consisting of (a) bidisperse mixtures of grains and (b) different monodisperse grains (one type for each barchan). We performed experiments in a water channel where grains of different sizes were poured inside forming two barchans that interacted with each other while filmed by a camera, and we obtained their morphology from image processing. We observed that a transient stripe appears over the dunes in cases of bidisperse mixtures, that interaction patterns vary with concentrations, and that different interactions exist when each barchan consists of different monodisperse grains. Interestingly, we found the conditions for a collision in which the upstream barchan is larger than the downstream one, and we propose a timescale for the interactions of both monodisperse and bidisperse barchans. Our results represent a new step toward understanding complex barchanoid structures found on Earth, Mars, and other celestial bodies. Plain Language Summary: Barchans are crescent‐shaped dunes commonly found in dune fields on Earth and other planetary environments, and it has been shown that their sizes are highly influenced by barchan‐barchan interactions. The composition of the granular bed can affect the observed patterns and sizes, but had not been investigated until now. Because those interactions take long times to be completed in gaseous environments (estimated in decades and millennia for dunes on terrestrial and Martian deserts, respectively) when compared with the aquatic case (of the order of minutes), we performed experiments in a water channel where two species of grains (in terms of size) were poured inside, forming two barchans that interacted with each other. We found different structures, including a collision in which the upstream barchan is the largest, and propose a timescale for the interactions of barchans. The identification of such timescale represents a new step for predicting the duration of barchan‐barchan interactions and, more generally, scaling the evolution of dune fields on Earth, Mars, and other planetary environments. Key Points: Two‐species barchan‐barchan interactions vary with the relative concentrations of each speciesWe found configurations distinct from the one‐species case, including a collision in which the upstream barchan is the largestWe propose a timescale for the interactions of both monodisperse and bidisperse barchans [ABSTRACT FROM AUTHOR]