The spectra of black hole binaries in the low/hard state are complex, with evidence for
multiple different Comptonisation regions contributing to the hard X-rays, in addition
to a cool disc component. However, it is difficult to constrain the shapes of these
components uniquely using spectral data alone. We show how additional information from
fast variability can break this degeneracy. In GX 339-4 we focus on the strong
variability features known as quasi-periodic oscillations (QPOs), and in Cygnus X-1 we
study the broad-band variability. In both sources, the results not only require
inhomogeneous Comptonization, but also allow us to constrain the spectral components. We
discuss our results in the context of propagating fluctuations through a hot flow, and
the implications for our picture of the accretion flow.