To make this type of mirror (which are also sometimes referred to as one-way mirrors), a thin layer of metal—usually aluminum—is applied to the front of a pane of glass. The layer is so thin that only half of the light that hits it is reflected back; the rest goes through the pane
For my two-way mirror i brought a piece of pesrpex instead of glass (much cheaper) and then brought a reflective film which i applied onto the perspex after washing it throughly with fairy liquid then squeegee the water off completely. Then i laid the foil on top to let it stick and cut to size.
When there is light on both sides the 2-way-mirror will act the same on both sides with neither being only a mirror or only see-through.