Dynein and its cofactor dynactin form a highly processive microtubule motor in the presence of an activating adaptor, such as BICD2. Different adaptors link dynein and dynactin to distinct cargoes. Here we use electron microscopy and single-molecule studies to show that adaptors can recruit a second dynein to dynactin. Whereas BICD2 is biased towards recruiting a single dynein, the adaptors BICDR1 and HOOK3 predominantly recruit two dyneins. We find that the shift towards a double dynein complex increases both the force and speed of the microtubule motor. Our 3.5 A resolution cryoelectron microscopy reconstruction of a dynein tail-dynactin-BICDR1 complex reveals how dynactin can act as a scaffold to coordinate two dyneins side-by-side. Our work provides a structural basis for understanding how diverse adaptors recruit different numbers of dyneins and regulate the motile properties of the dynein-dynactin transport machine.