[mikrofon]

Correct me if I'm wrong, but even if Earth & its "twin" were the only planets in the System, their orbits would have to be ABSOLUTE mirrors of each other —not the same path— for them to always be in Opposition (and mutually invisible)… And the more elliptical the orbit(s), the greater the angular separation from the Sun would be at any one epoch. 

[eclecticEel]

They wouldn’t have to “always” be in opposition; the question is how long a planet could remain on the other side of the sun without being detectable.