  Breaks in the radial luminosity profiles of galaxies have been until now
mostly studied averaged over discs. Here we study separately breaks in thin and
thick discs in 70 edge-on galaxies using imaging from the Spitzer Survey of
Stellar Structure in Galaxies. We built luminosity profiles of the thin and the
thick discs parallel to midplanes and we found that thin discs often truncate
(77%). Thick discs truncate less often (31%), but when they do, their break
radius is comparable with that in the thin disc. This suggests either two
different truncation mechanisms - one of dynamical origin affecting both discs
simultaneously and another one only affecting the thin disc - or a single
mechanism that creates a truncation in one disc or in both depending on some
galaxy property. Thin discs apparently antitruncate in around 40% of galaxies.
However, in many cases, these antitruncations are an artifact caused by the
superposition of a thin disc and a thick disc with the latter having a longer
scale length. We estimate the real thin disc antitruncation fraction to be less
than 15%. We found that the ratio of the thick and thin stellar disc mass is
roughly constant (0.2<M_T/M_t<0.7) for circular velocities v_c>120 km/s, but
becomes much larger at smaller velocities. We hypothesize that this is due to a
combination of a high efficiency of supernova feedback and a slower dynamical
evolution in lower-mass galaxies causing stellar thin discs to be younger and
less massive than in higher-mass galaxies.
