Mixed-species association is common among primates but usually loose. Emperor and saddleback tamarins go further: the same two groups may stay together across seasons, sharing sleeping-site departures, coordinating travel routes and answering each other's alarm calls. Each species responds to the other's calls as though they were its own.
The arrangement works because the species do not overlap fully. Saddlebacks tend to forage lower, probing bark, tangles and knot holes for prey, while emperors work higher in the canopy and take more insects from foliage. Predation pressure is severe for animals of this size, and a larger combined group is harder to surprise.
The moustache is the feature everyone remembers and the one with the least evidence behind it. The old story that it was named after the German emperor Wilhelm II appears to have begun as a joke, and no functional explanation of the moustache itself has been demonstrated; species recognition in mixed troops is a plausible but untested candidate.