Anent Graham White's suggestions,
Two possible names are "liminal" (from limen, a doorway), or "bifrontal" (from frons, which means face: one of the titles of Janus is Janus bifrons). I kind of like liminal, because it emphasises the function of the twofacedness, rather than simply the fact that the object is twofaced.
Perhaps "liminal" suggests two-facedness to some, but not to Merriam- Webster ( http://www.merriam-webster.com/dictionary/liminal ), who believe it stands for 1: of or relating to a sensory threshold; 2: barely perceptible; or 3: of, relating to, or being an intermediate state, phase, or condition : in-between, transitional <in the liminal state between life and death — Deborah Jowitt>. And never mind that "two-facedness" in common parlance has to do with a person's being (not liminal, but) deceitful, insincere, or hypocritical. OtOH, liminal does accept prefixes nicely, as: subliminal, supraliminal :-) . As for "bifrontal", often one is faced (sorry :-) ) with an object having far more "fronts" than just two. Good old 2 = {0, 1}, for example, is: a set, a pointed set, a bi-pointed set, a poset, a poset with top, a poset with bottom, a compact T2 space, a pointed compact T2 space, an abelian group, a meet-semilattice, a frame (though others will insist I should be saying "locale"), a Boolean Ring, a Boolean Rng, and much much more. Does the prefix "bi-" really adequately capture the potential of having all that many ... umm ... hate to use this term ... personalities? Cheers, -- Fred "it takes two to tango" Linton, (now [re]tiring from -- sitting out the rest of -- this year's dance) [For admin and other information see: http://www.mta.ca/~cat-dist/ ]