We present high time-resolution multicolour optical observations of the quiescent X-ray transients GRS 1124-684 (=GUMus) and Cen X-4 (=V822Cen) obtained with ULTRACAM. Superimposed on the secondary stars' ellipsoidal modulation in both objects are large flares on time-scales of 30-60min as well as several distinct rapid flares on time-scales of a few minutes, most of which show further variability and unresolved structure. Not significant quasi-periodic oscillations are observed and the power density spectra of GRS 1124-684 and Cen X-4 can be described by a power law. From the colour-colour diagrams of the flare events, for GRS 1124-684 we find that the flares can be described by hydrogen gas with a density of NH ~ 1024nucleonscm-2, a temperature of ~8000K and arising from a radius of ~0.3Rsolar. Finally we compile the values for the transition radius (the radius of the hot advection-dominated accretion flow) estimated from quasi-periodic oscillations and/or breaks in the power density spectrum for a variety of X-ray transients in different X-ray states. As expected, we find a strong correlation between the bolometric luminosity and the transition radius. Based on observations made at the European Southern Observatory, Paranal, Chile (ESO programme 075.D-0193). E-mail: tsh [at] ll.iac.es (tsh[at]ll[dot]iac[dot]es)