Just looked it up. Pretty sure you're right about the hyphen, but I couldn't really find a clear answer. It has a hyphen in the dictionary, so I'm going to assume that's correct.
A "Pair of two" is how the glasses are constructed. One way mirrors are actually layers of materials, with a thin half-silver coating acting as the reflective property. When he got the reflective property enhanced, they added an extra coating, therefore making it effectively two one way mirrors per lens. Should have made that more clear. The word effectively used to be in there, but I took it out because it made the sentence cluttered and awful. Sorry for the bad explanation.

The story is in a more contemporary setting than all of those. Although, I haven't read half of them, so I'm not sure.

and thank you for your comment!