  The multitime probability distributions obtained by repeatedly probing a
quantum system via the measurement of an observable generally violate
Kolmogorov's consistency property. Therefore, one cannot interpret such
distributions as the result of the sampling of a single trajectory. We show
that, nonetheless, they do result from the sampling of one pair of
trajectories. In this sense, rather than give up on trajectories, quantum
mechanics requires to double down on them. To this purpose, we prove a
generalization of the Kolmogorov extension theorem that applies to families of
complex-valued bi-probability distributions (that is, defined on pairs of
elements of the original sample spaces), and we employ this result in the
quantum mechanical scenario. We also discuss the relation of our results with
the quantum comb formalism.
