We present an atom interferometry technique in which the beam splitter is split into two separate operations. A microwave pulse first creates a spin-state superposition, before optical adiabatic passage spatially separates the arms of that superposition. Despite using a thermal atom sample in a small (600 μm) interferometry beam, this procedure delivers an efficiency of 99% per ℏk of momentum separation. Utilizing this efficiency, we first demonstrate interferometry with up to 16ℏk momentum splitting and free-fall limited interrogation times. We then realize a single-source gradiometer, in which two interferometers measuring a relative phase originate from the same atomic wave function. Finally, we demonstrate a resonant interferometer with over 100 adiabatic passages, and thus over 400ℏk total momentum transferred.