Both panels now sit on the same time axis (logarithmic, in units of the reference sample’s characteristic time). One sweep line crosses both, so the two markers always move together — there is no “faster ball”. What differs is where each curve sits on that axis: the flash rear-face rise happens far to the left, the steady-state approach far to the right. Make the sample thicker or slower and watch both curves slide right together — that is the L²/α law you can see.
Both panels share one logarithmic time axis in units of the reference sample’s L²/α, so a single sweep line serves both. Curves are normalized teaching solutions: a lumped exponential approach to equilibrium and the ideal one-dimensional adiabatic Parker rear-face series.