You're treading on a peculiar interest of mine, Cassius! The double herm was the reason I bought Bernard Frischer's book.
"Herm" in this case is short for Hermes, who was the figure chiefly represented in early herm statues. The typical herm was a standing stone with a bust carved into the topmost portion. In the case of Hermes in particular, the rest of the statue would be left squared off all the way down, apart in some cases from a conspicuous set of genitals at the appropriate location.
Hermes was the patron god of messengers, merchants and travelers, and–by extension–roads, highways, and crossings. The herm statue was in some places used to mark roads, in some places to mark milestones, and in others to mark boundaries (The Romans had their own patron god of boundaries, Jupiter Terminus, a statue of whom would be placed on property lines and propitiated by both neighbors in a special ritual on Terminalia every spring).
How the herm statue came to be sculpted with two heads facing opposite is an interesting question. There was another god, Janus, with a face on either side of his head–he presided over the new year, with one aspect facing to the future and one to the past. In Hermes' case, there was a cultural boundary line just as important as those of time and property; the diad between male and female. Aphrodite was often chosen as the figure to complement him.
In other statues the twin figures are an old man and a youth; the key feature in all of these artistic expressions is the curious interplay of limitation and continuity.
Metrodorus, who would certainly have succeeded Epicurus had he survived him, represented continuity–the master/student relationship, the succession of the scholarchs, etc.
I disliked the double herm at first sight, but I'm beginning to grasp its meaning better by seeing it through Greek and Roman eyes.
it's a pretty quick process.