  Synchronization, the temporal coordination of coupled oscillators, allows
fireflies to flash in unison, neurons to fire collectively and human crowds to
fall in step on the London Millenium bridge. Here, we interpret active (or
self-propelled) chiral microswimmers with a distribution of intrinsic
frequencies as motile oscillators and show that they can synchronize over very
large distances, even for local coupling in 2D. This opposes to canonical
non-active oscillators on static or time-dependent networks, leading to
synchronized domains only. A consequence of this activity-induced
synchronization is the emergence of a `mutual flocking phase', where particles
of opposite chirality cooperate to form superimposed flocks moving at a
relative angle to each other, providing a chiral active matter analogue to the
celebrated Toner-Tu phase. The underlying mechanism employs a positive feedback
loop involving the two-way coupling between oscillators' phase and
self-propulsion, and could be exploited as a design principle for synthetic
active materials and chiral self-sorting techniques
