 Consider an example from physics. The Standard Model of particle physics treats quarks as point-like particles that have no internal structure; they are mereological atoms. But quarks do not exist independently; they come in groups of two or three, such as in the case of mesons, protons, and neutrons. So, you do not get freely existing quarks, mereological atoms though they may be. There seems to be at least a weak type of symmetric existential dependence between the three quarks that compose, say, a given proton. In other words, the three quarks that compose a proton are symmetrically dependent on each other for their existence and perhaps even for their identity or essence, though this is much more controversial (see the entry on  ontological dependence  for discussion on identity-dependence and essential dependence).