  We present an atom interferometry technique in which the beamsplitter 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
($600 \, \mu$m) interferometry beam, this procedure delivers an efficiency of
$99\%$ per $\hbar k$ of momentum separation. Utilizing this efficiency, we
first demonstrate interferometry with up to $16\hbar k$ 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 wavefunction. Finally, we demonstrate a resonant
interferometer with over 100 adiabatic passages, and thus over $ 400 \hbar k$
total momentum transferred.
