There is in Nara’s Asuka village, a stone called “Nimen-seki” with the two faces of good and evil carved back to back. It is one of the ancient strange stones in the now famous and much visited historic village. The stone is located at Tachibanadera in Asuka-mura, Takaichi-gun. Tachibanadera is a temple of Tendai Sect. Its formal name is “Butto-zan Jogu Oin Bodaiji Temple.” According to the temple tradition, the name “Tachibana (mandarin orange tree)” refers to the story that Emperor Suinin sent Tajima-mori to Hitachi province (present Ibaraki Prefecture) to fetch the everlasting fruit of the mandarin orange tree, and who returned with some which were planted at this place. The temple site is also famous for the other historical reason that it was where Emperor Yomei’s detached palace, Tachibana Palace, was located and where Prince Shotoku was born. The temple is one of 7 temples founded by Prince Shotoku. Source: NIPPON-KICHI
The two-faced statue may have been probably a locally commissioned (Sogdian / Kushan-looking?) variant of the well-known Roman-Janus or the Iranian-Zoroastrian Vayu versions:
Janus Bifrons at the Vatican Museum
“… Janus was originally nothing but the god of doors. That a deity of his dignity and importance, whom the Romans revered as a god of gods and the father of his people, should have started in life as a humble, though doubtless respectable, doorkeeper appears very unlikely. So lofty an end hardly consorts with so lowly a beginning. It is more probable that the door (janua) got its name from Janus than that he got his name from it. This view is strengthened by a consideration of the word janua itself. The regular word for door is the same in all the languages of the Aryan family from India to Ireland. It is dur in Sanscrit, thura in Greek, tür in German, door in English, dorus in old Irish, and foris in Latin. Yet besides this ordinary name for door, which the Latins shared with all their Aryan brethren, they had also the name janua, to which there is no corresponding term in any Indo-European speech. The word has the appearance of being an adjectival form derived from the noun Janus. I conjecture that it may have been customary to set up an image or symbol of Janus at the principal door of the house in order to place the entrance under the protection of the great god. A door thus guarded might be known as a janua foris, that is, a Januan door, and the phrase might in time be abridged into janua, the noun foris being understood but not expressed. From this to the use of janua to designate a door in general, whether guarded by an image of Janus or not, would be an easy and natural transition. 8
If there is any truth in this conjecture, it may explain very simply the origin of the double head of Janus, which has so long exercised the ingenuity of mythologists. When it had become customary to guard the entrance of houses and towns by an image of Janus, it might well be deemed necessary to make the sentinel god look both ways, before and behind, at the same time, in order that nothing should escape his vigilant eye. For if the divine watchman always faced in one direction, it is easy to imagine what mischief might have been wrought with impunity behind his back. This explanation of the double-headed Janus at Rome is confirmed by the double-headed idol which the Bush negroes in the interior of Surinam regularly set up as a guardian at the entrance of a village. The idol consists of a block of wood with a human face rudely carved on each side; it stands under a gateway composed of two uprights and a cross-bar. Beside the idol generally lies a white rag intended to keep off the devil; and sometimes there is also a stick [as is observable in the nimenseki statue] which seems to represent a bludgeon or weapon of some sort. Further, from the cross-bar hangs a small log which serves the useful purpose of knocking on the head any evil spirit who might attempt to pass through the gateway. Clearly this double-headed fetish at the gateway of the negro villages in Surinam bears a close resemblance to the double-headed images of Janus which, grasping a stick in one hand and a key in the other, stood sentinel at Roman gates and doorways; and we can hardly doubt that in both cases the heads facing two ways are to be similarly explained as expressive of the vigilance of the guardian god, who kept his eye on spiritual foes behind and before, and stood ready to bludgeon them on the spot.”
According to Wikipedia article on Janus, he was “likely the most important god in the Roman archaic pantheon. He was often invoked together with Iuppiter(Jupiter)” and :
Modern scholars have conjectured that it derives from the Indo-European root meaning transitional movement (cf. Sanskrit “yana-” or Avestan “yah-“, likewise with Latin “i-” and Greek “ei-“.). Iānus would then be an action name expressing the idea of going, passing, formed on the root *yā- < *y-eð2– theme II of the root *ey- go from which eō, ειμι.
The function of ‘god of beginnings’ has been clearly expressed in numerous ancient sources, among them most notably Cicero, Ovid and Varro. As a god of motion Janus looks after passages, causes actions to start and presides over all beginnings, and since movement and change are bivalent, he has a double nature, symbolised in his two headed image. He has under his tutelage the stepping in and out of the door of homes, the ianua, which took its name from him, and not viceversa. Similarly his tutelage extends to the covered passages named iani and foremost to the gates of the city, including the cultic gate of theArgiletum, named Ianus Geminus or Porta Ianualis from which he protects Rome against the Sabines. He is also present at the Sororium Tigillum, where he guards the terminus of the ways into Rome from Latium. He has an altar, later a temple near the Porta Carmentalis, where the road leading to Veii ended, as well as being present on the Janiculum, a gateway from Rome out to Etruria.
The connection of the notions of beginning (principium), movement, transition (eundo), and thence time has been clearly expressed by Cicero. In general, Janus is at the origin of time as the guardian of the gates of Heaven: Jupiter himself can move forth and back because of Janus’s working. In one of his temples, probably that of Forum Holitorium, the hands of his statue were positioned to signify the number 355 (the number of days in a year), later 365, symbolically expressing his mastership over time. He presides over the concrete and abstract beginnings of the world, such as religion and the gods themselves, he too holds the access to Heaven and to other gods: this is the reason why men must invoke him first, regardless of the god they want to pray or placate. …
... in IE religions there is an introducer god (as Vedic Vâyu and Roman Janus) and a god of ending, a nurturer goddess and a genie of fire (as Vedic Saraswati and Agni, Avestic Armaiti, Anâitâ and Roman Vesta) who show a sort of mutual solidarity: the concept of ‘god of ending’ is defined”.
From the Wikipedia article “Vayu“, we note another important icon deity that has widespread influence along the Silk Road from the Greek god (Boreas?) found in Hadda, to the Kizil wind deity to the Japanese Fujin wind god:
“Vāyu-Vāta or Vāta-Vāyu (IPA: ʋɑːyu-ʋɑːt̪ə) is the Avestan language name of a dual-natured Zoroastrian divinity of the wind (Vayu) and of the atmosphere (Vata). The names are also used independently of one another, with ‘Vayu’ occurring more frequently than ‘Vata’, but even when used independently still representing the other aspect.
The entity is simultaneously angelic and demonic, that is, depending on the circumstances, either yazata – “worthy of worship” – or daeva, which in Zoroastrian tradition is a demon. Scripture frequently applies the epithet “good” when speaking of one or the other in a positive context.
In Zurvanism (Zurvanite Zoroastrianism, a now-extinct form of Zoroastrianism), Vata-Vayu represented two facets of the quaternary Zurvan. In this arrangement, Vata-Vayu represented “space” while the other two facets represent “time.”
Vayu-Vata has Indo-Iranian roots, and has the same name in historical Vedic religion.”