What similarities are present between Macbeth and Lady Macbeth other than their ambition?

In Shakespeare's Macbeth, feelings of guilt are another trait that Macbeth and Lady Macbeth have in common, though they feel them at different times.

In fact, the Macbeths experience role reversals during the course of the play.

Macbeth feels guilt before murdering Duncan.  This is itself a role reversal, as Macbeth reveals emotions usually associated with females.  In Act 1.7 Macbeth first worries about the consequences of murdering King Duncan, but he also feels badly because Duncan is his kinsman, and Macbeth is his subject.  He also feels badly because he is, at the moment, Duncan's host, and as such he should be concentrating on protecting his guest, not assassinating him.  He feels badly because Duncan has been a "meek" king, and has led the country blamelessly. 

Lady Macbeth, before the assassination, doesn't worry about the consequences, and seems to feel no guilt.  She pleads to be unsexed--made more like an aggressive male--and manipulates her husband into killing Duncan.  When he obsesses over the blood on his hands immediately after killing Duncan, she ridicules him and tells him "A little water clears us of this deed" (2.2.70).

Yet, once the assassination occurs, Macbeth loses all sense of guilt, and Lady Macbeth suffers so much from guilt that she ends up a suicide.  While Macbeth kills the grooms, orders the killings of Banquo and Fleance and the slaughter of Macduff's family, Lady Macbeth suffers.  She sleepwalks, ringing her hands (obsessing over imaginary blood on her hands) in Act 5.1.  And she commits suicide later in the play. 

Ambition does unite the Macbeths, but so does their guilt.