Conservation of energy says to me that you cannot have full transmission in both directions and get any reflection, additionally with real glass there will be some absorption.

If by two-way-mirror, you mean a partially reflective sheet of "glass", then a shallow box with BB's Tricky glass shader should be the starting point.

With the tricky glass, where you mix a GlassBSDF node with the TransparentBSDF node, default Glass has its own inherent Index of Refraction, about 1.45. That number, essentially, is what determines how much reflection and refraction happen at the glass surface. Push the number up to the point BB determined emulates metals, between 10 and 20, and you get a more or less completely reflective surface. Come back down to the levels of diamond, 2.1 and you get much more surface reflection than ordinary glass. Plugin 5 as the IOR, and you get something that gives roughly equal transmission and reflection.

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