Hi,
What about having those 2 planets in synchronized opposed orbits? This way for a very long time (evolution) the other planet is behind the sun, preventing any direct observation and also early radio communications. Even the first stage of space exploration around respective home lands would not give direct knowlege of the other world.
I guess such orbits would require some tricky explaination has giant planets (jupiter like) gravity perturbation could desync orbits, but that could as well be the trigger to slowly make the 2 civilisation aware of each other. You even got the title for it : "Behind the sun"
Orbits could also be slightly different, making the 2 planets visible for 1000 years every 10000 years (I guess you can do the math on orbits to get the numbers you need), the book then explaining "cheap" travel to the other world only occurs once in a while and the 2 species descending from on common ancestor, thus explaining why they are technologically close.
Sorry for the quite messy ideas there... just found the topic interesting and wrote it on the fly...
