How is the relationship between Macbeth and Lady Macbeth being demonstrated?

The relationship between Macbeth and Lady Macbeth is first demonstrated by Macbeth's letter to her.  In it, he calls her his "dearest partner of greatness" and says that he wanted to write to her, even though he would soon be home, so she would not miss the chance to "rejoic[e] [over] what greatness is promised [her]"; he knows she will be pleased to learn that she will be queen (1.5.11, 12-13).  Thus, unlike many loveless arranged marriages of this time, it seems that the Macbeths genuinely do care about one another.  To call her his partner of greatness is to raise her to the level of his equal, something that was not common of eleventh century marriages.  When he returns home, Macbeth calls his wife his "dearest love" (1.5.67), and we can see that they understand one another.  She can read his face, and they seem to understand what the other is thinking without having to speak their thoughts aloud.  They seem to be a good match.

Later in the play, however, Macbeth begins to make plans and take steps without consulting his wife, and so it seems as though they are no longer really "partners" in his mind.  He arranges for the murders of Banquo and Fleance, and he purposely keeps her in the dark, saying, "Be innocent of the knowledge, dearest chuck, / Till thou applaud the deed" (3.2.51-52).  She knows, then, that he is acting without her now, and by the next (and last) time we see her, she is sleepwalking in Act 5, Scene 1.  It seems as though she regrets the monster she helped to create because she imagines that she cannot remove the blood from her hands, and she laments the death of Lady Macduff, another victim of the plans Macbeth made without her.