  We consider the problem of the general assessment of non-Markovianity in
multipartite open quantum systems. To this end, we elaborate on a previously
introduced non-Markovian measure to formulate a finite and additive quantifier
with appropriate asymptotic behavior. We then study the dynamics of two
two-level atoms in a resonant cavity as a function of their interatomic
distance using an exact treatment. We observe a critical behavior in the
non-Markovianity in the form of a Markovian to non-Markovian transition at a
finite distance and a discontinuity in the limit of infinitely close atoms.
Additionally, in this limit of highly correlated dynamics, non-Markovianity can
emerge when the size of the system exceeds a critical threshold.
