In Macbeth, what two main faults does Malcolm pretend he has when he's talking to Macduff?
The scene you are referring to is when Macduff comes to Malcolm to enlist his help to overthrow Macbeth from the throne. Malcolm is wary of Macduff's true intentions and therefore tests him by pretending to have too many vices to be of anyhelp to Macduff.
Check out Act 4 scene 3, and notice how Macduff says he has "vices" too. Under this section, his two main vices are:
Lust - he argues that his lust could not be quenched by all of Macduff's wives, daughters and old women and young women. He would go to any lengths to try to satisfy this lust.
Greed - Malcolm notes that he would kill the landowners under him in order to get their homes and wealth.
He then lists qualities a king should hold and argues that he does not possess any of them. These include "justice, verity, temperance, stableness, bounty, perseverance, mercy, lowliness, devotion, patience, courage, fortitude" (4.3.94-96).