The language evolved: it became OK to refer to an emperor and a
Caesar together as
Augusti, though if you preferred you could
still say
AVG ET
CAES!
The first indubitable example:
VICTORIA AVGVSTORVM on a
rare sestertius of
Maximinus Thrax,
BMC 183, pl. 39, referring to
Maximinus the
Augustus and
his son
Maximus Caesar, who never became
Augustus. The
type itself even clearly distinguishes their rank:
Maximinus is laureate, but
Maximus bare-headed.
Another possible example: SALVTI
AVGG and
VICT AVGG COS II P P on coins of
Septimius Severus as
IMP X in late 197/early 198, a little before he made
Caracalla his co-Augustus.
Caracalla did, however, already have the title "
Imperator Destinatus".
A related phenomenon:
provincial coins begin to call successors not just
Caesar, but
Caesar Augustus,
KAICAP CEBACTOC. So on Alexandrian coins from
Diadumenian on, and in papyri from
Geta on: see
Vogt, Alexandrinische Münzen, p. 174.
You're not alone, however, in finding
AVGG strange for
Philip I in 245: that word made
Pink misdate the
type to 247, after
Philip II became
Augustus, referring it to the emperors' return to
Rome from their Danubian campaign!
