We propose an experiment to test whether the one-way speed of light is isotropic, without relying on clock synchronisation between separated stations. Two identical stations separated by a fixed distance exchange laser beams through complementary shutters driven by independent atomic clocks. The phase relationship between the two stations' light maxima directly encodes any asymmetry in one-way light speed. No synchronisation signal is required and the measurement is not circular. Relative clock drift between stations is monitored by observing whether the inter-station timestamp offset remains constant over the measurement interval. This offset is not interpreted as a one-way propagation time and does not constitute a synchronisation convention -- it serves solely as a stability diagnostic. The known frequency precision of atomic clocks, combined with regular monitoring of this offset, provides sufficient warranty of system integrity. The experiment is sensitive to any preferred-frame velocity component along the baseline. The true velocity of the apparatus through such a frame -- if one exists -- is unknown. Earth's orbital velocity of approximately 30 km/s is a lower bound only; contributions from the motion of the solar system and galaxy relative to other astronomical reference points could raise the true figure substantially. A null result would constrain preferred-frame theories. A non-null result would warrant careful independent replication.