Hi Faldage,
the other of two
this is indeed the primary meaning, but the signification has grown beyond these limits. "Perseus Word Study Tools" has the following:
A. Another of a class = alius (as opp. to one's self, to another); subst., another, a neighbor, a fellow-creature, ho pelas (so sometimes heteros, Xen. Cyr. 2, 3, 17); cf. Ochsn. Eclog. 90 and 458 (alter designates the similarity of two objects; alius a difference in the objects contrasted).
(found after my first posting)