How are the traditional stereotypes of men and women reversed in chapter 1 & 2 of Part 2.
If you are talking about traditional stereotypes from our own society (rather than anything within the 1984 society), they are reversed in the way that Winston and Julia behave. Specifically, she is the aggressor and he is the one being pursued.
In our society, it is stereotypically the man's role to pursue a woman that he is interested in. But in the chapters you mention, it is Julia who is pursuing. It is Julia who puts the note in Winston's hand. It is Julia who specifies when and where they will get together. It is Julia who has had affairs before and it is Julia who is in control of their encounter.
So, this all goes against our sex roles -- the ones that say women should be more passive in romantic/sexual matters.