I have a valid Italian schengen in an expired passport till the 19th September. Have a new Italian Schengen that starts on 20th September in the new passport. I have to travel on the 16th and be there till the 29th. The embassy infact issued visas back to back, but is this possible that I enter on one and exit on the other in a new passport?? please advice
Yes, this is fine, and it is indeed why the visas were issued back to back. The Schengen system does not place any importance on the identity of the visa used by a traveler to enter or exit; it's only important that a visitor have a valid visa authorizing presence in the Schengen area (unless exempted by reason of nationality or otherwise).
It's also necessary to comply with the 90/180 rule, which applies across visas. For example, if 19 September is your 90th day in the Schengen area over the previous 178 days (or fewer), then you are not permitted to remain until 20 September.