  The stellar disk of the Milky Way shows complex spatial and abundance
structure that is central to understanding the key physical mechanisms
responsible for shaping our Galaxy. In this study, we use six very high
resolution cosmological zoom simulations of Milky Way-sized haloes to study the
prevalence and formation of chemically distinct disc components. We find that
our simulations develop a clearly bimodal distribution in the $[\rm \alpha/Fe]$
-- $[\rm Fe/H]$ plane. We find two main pathways to creating this dichotomy
which operate in different regions of the galaxies: a) an early ($z>1$) and
intense high-$\rm[\alpha/Fe]$ star formation phase in the inner region
($R\lesssim 5$ kpc) induced by gas-rich mergers, followed by more quiescent
low-$\rm[\alpha/Fe]$ star formation; and b) an early phase of
high-$\rm[\alpha/Fe]$ star formation in the outer disc followed by a shrinking
of the gas disc owing to a temporarily lowered gas accretion rate, after which
disc growth resumes. In process b), a double-peaked star formation history
around the time and radius of disc shrinking accentuates the dichotomy. If the
early star formation phase is prolonged (rather than short and intense),
chemical evolution proceeds as per process a) in the inner region, but the
dichotomy is less clear. In the outer region, the dichotomy is only evident if
the first intense phase of star formation covers a large enough radial range
before disc shrinking occurs; otherwise, the outer disc consists of only
low-$\rm[\alpha/Fe]$ sequence stars. We discuss the implication that both
processes occurred in the Milky Way.
