A matched set of two full-length mirrors, at least the height and width of an average medium-sized humanoid.
The two-way mirrors function as somewhat-secret, somewhat-portable teleportation circles. An individual may step through one mirror as though it were intangible, and find themselves exiting through the other mirror, like a doorway.
The mirrors function as doorways from one to the other so long as both mirrors remain on the same plane of existence. The mirrors can be placed within a Bag of Holding to no ill effect, but for the duration that one of the mirrors is within a Bag of Holding, the other mirror cannot function, as the first mirror is considered to be in a pocket dimension and thus no longer on the same plane.
If a Two-Way Mirror is encountered by a player that does not know what it is, a DC 17 arcana check will alert them that the mirror is magical. A DC 17 perception check on the mirror will allow them to hear a muffled version of whatever is audible in the location of the other mirror.