OK guys here is an update,
Me and my wife both appeared for our VISA interview and fortunately both of us got thru.
Since mine was a VISA renewal case and her's was a regular (fresh) H4 VISA, we could not club the appointments together but appeared for the interview seperately.
Following are the answers to my own questions originally posted
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I hold valid H1-B VISA which currently stammped, I have a petition I-797 for current VISA which is valid till SEPT 2012.I recently applied for an extension and got it approved and received a new I-797 valid from SEPT 2012 to DEC 2014 for the same employer.
I got married this year and now I am planning apply for H4 visa for my wife. But I have few doubts, any advise in this regards is really appriciated.
1. Should my wife need to carry both I-797s?
Ans: Yes
2. Which I-797 do should mention under DS-160/Visa appointment with VFS?
Ans: I used my existing I-797 valid until SEPT 2012
3. How will it affect my wife's I-94 dates?
Ans: My wife got her I-94 dated until Dec 2014 at the port of entry.
4. which petition number will be mentioned in her H4 stamp?
Ans: Both the petition numbers are mentioned
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Hope this helps somebody in the same boat I was.