Sheridan LeFanu's classic work about a lesbian vampire, Carmilla, describes vampires thus:
In modern parlance, water is considered to be a bane for vampires, so when I saw that phrase in Carmilla, I wondered if it was a typo, but it is used again later in that chapter:"How they escape from their graves and return to them for certain hours very day, without displacing the clay or leaving any trace of disturbance in the state of the coffin or the cerements, has always been utterly inexplicable. The amphibious existence of the vampire is sustained by daily renewed slumber in the grave. Its horrible lust for living blood supplies the vigor of its waking existence."
"[Vondenburg] has left a curious paper to prove that the vampire, on expulsion from its amphibious existence, is projected into a far more horrible life, and he resolved to save his once beloved Mircalla from this."
My standard dictionaries provide definitions only involving a dual existence on land and in the water, but I pulled out the OED to find some archaic usages. The third definition is simply "having two lives," with citations from the 1640s to the 1840s, including Coleridge.
The justification for the usage is apparent from the etymology:
From Ancient Greek ἀμφίβιος (amphíbios). From ἀμφί (amphí, "two sides") + βίος (bíos, “life”).
You learn something every day.