  The most unstable quantum states and elementary particles possess more than a
single decay channel. At the same time, it is well known that typically the
decay law is not simply exponential. Therefore, it is natural to ask how to
spot the non-exponential decay when (at least) two decay channels are opened.
In this work, we study the tunneling phenomenon of an initially localized
particle in two spatially opposite directions through two different barriers,
mimicking two decay channels. In this framework, through a specific quantum
mechanical examples which can be accurately solved, we study general properties
of a two-channel decay that apply for various unstable quantum states (among
which also for unstable particles). Apart from small deviations at early times,
the survival probability and the partial tunneling probability along the chosen
direction are very well described by the exponential-decay model. In contrast,
the ratios of the decay probabilities and probability currents are evidently
not a simple constant (as they would be in the exponential limit) but display
time-persisting oscillations. Hence, these ratios are optimal witnesses of
deviations from the exponential decay law.
