Atah Echad Vi-Shimcha Echad

On Shabbos mincha we say אתה אחד ושמך אחד - You are One and Your name is One. "Hashem is One" means that he is the only reality, the only true existence. We seem real to ourselves but we are essentially a figment of our common imaginations [Nefesh Hachaim]. That being the case, we seemingly contradict ourselves ["seemingly" because we can't contradict ourselves when we really don't exist...:)]. אתה implies that there is a "me". If I say "you" that means that there is a "me". So how can we say אתה  meaning that there is a reality outside of you [i.e. me] and yet you are אחד the one and only reality? [I'll bet this question has bothered you for a long time....]

The answer is that indeed Hashem is the only true reality but he makes a place for us in order that ושמך אחד - His "Name" should be spread to the world. The only reason that there is הסתר [hiddenness] is to allow us to search for and find Him.

So the אתה אחד [an apparent separate reality that is truthfully only one reality] is so that ושמך אחד - His name is made One and people realize on their own that His is the only true one.

[Based on Rav Dessler] 

לזכות נעכא גיטל בת רחל אסתר זרעא חייא וקיימא זרעא די לא יפסוק ודי לא יבטול מפתגמי אורייתא