Turbine assemblies and related turbocharger systems having direct turbine interfaces are provided. One exemplary turbine assembly includes a first turbine housing having an outlet portion defining a fluid outlet of a first turbine and a second turbine housing having an inlet portion defining a fluid inlet of a second turbine, wherein at least a portion of the outlet portion radially surrounds at least a portion of the inlet portion to provide a direct interface from the fluid outlet of the first turbine to the fluid inlet of the second turbine in an axial direction.