  We report on observations of PSR B1508+55's scintillation at the Effelsberg
100-m telescope spanning from early 2020 to early 2022. In the autumn of 2020,
close to the time the pulsar was predicted to cross echoes in its pulse
profile, a sudden transition in the scintillation arcs from peculiar
stripe-like features to parabolic arclets was observed. To infer a geometric
model of the scattering we measure the effects of the annual velocity curve of
Earth, of the relative movement of the line of sight, and of the projection of
points on a second scattering screen and develop novel methods to do so. The
latter phenomenon was discovered by this study and strongly indicates a
two-screen scattering geometry. We derive an analytical two-screen model and
demonstrate in a Markov Chain Monte Carlo analysis as well as simulations that
it can be successfully applied to explain the observations by interpreting the
transition as a change of relative amplitudes of images as well as a shift in
the orientation of anisotropy. The collection of methods we demonstrate here is
transferable to other pulsars with the potential to strongly improve
constraints on scattering models.
