In Macbeth, how should I compare and contrast Lady Macbeth's two soliloquies in Act 1?
I am a bit confused by your question because there is only one soliloquy by Lady Macbeth in Act I - in scene 5, so I will focus on analysing that for you.
When we first meet Lady Macbeth in Act I Scene 5 we are presented with a woman who is in every way harder, harsher and firmer than her husband, who she sees as being "too full o'the milk of human kindness" - a definite negative in her eyes. She sees it as her role to use the "valour of [her] tongue" to persuade him, and in dramatic language mobilises her whole being, even sacrificing her femininity, to achieve this purpose. She is presented as a ruthless and committed woman who is far more ambitious than her husband. The force of mind and hardness vital for an assassination is shown to come from her. We are also forced to see a comparison between Lady Macbeth and the witches. When she invokes the dark spirits to "unsex me" and "fill me" with "direst cruelty" there is a complete betrayal of humanity and femininity and a colossal abandonment of self to evil. This soliloquy of Lady Macbeth's is full of imperatives ("come", "Fill" etc) gives her speech added urgency and determination. It is interesting to note that when her husband arrives she greets him in the same way as the witches did in Act I scene 3. The messenger announcing the arrival of the king is a nice touch, as it is juxtaposed to Lady Macbeth's plans for him.
Publicly she acts innocently, but in private we see her for the evil, manipulative character that she is. She has total control of the situation and Macbeth stands in awe at her discipline and force of will. Look at Act I Scene 7 for an example of Lady Macbeth forcing her husband to accomplish her ends cajoling him and flattering him by turns to accomplish the deed.
Hopefully this will give you some ideas of how to analyse other parts of the play. Good luck!
Actually, in Act I, scene v, Lady Macbeth does have two soliloquies, broken up by the entrance and exit of a Messenger. The first one (lines 1-30) includes the reading of a letter from Macbeth. The second (lines 39 - 55) ends with Macbeth's entrance into the scene.
The first soliloquy is important because, in this soliloquy she is cluing the audience in to her take on Macbeth, and what she considers to be his weakness --
...I fear thy nature,
It is too full o'the milk of human kindness.
That wouldst thou holily; woulst not play false...
...that which...thou dost fear to do.
She seems, in this soliloquy, a somewhat ambitious but very female wife. She can only help to influence Macbeth by "pour[ing her] spirits in [his] ear," and cannot effect the act itself.
However, once the Messenger brings the news that Duncan will be at the Macbeth's castle that night, she goes into action. Gone is any seeming hesitation about her role and she calls upon supernatural forces to aid her:
...Come you spirits
That tend on mortal thoughts, unsex me here
And fill me from the crown to the toe top-full
Of direst cruelty....Come thick night,
And pall thee in the dunnest smoke of hell,
That my keen knife see not the wound it makes.
By this soliloquy, she suggests to the audience that she will take matters in to her own hands, her own knife shall be the one that strikes the evil blow that makes Macbeth king.