2 Entries Schengen Visa

I mentioned 2 entries requested in my visa application form for Italy (The first trip), Is the duration of stay going to be per entry or total amount of days for both trips?

The duration of stay covers both trips. You can use a single visa for both trips only if all of the following are true:

• The number of entries is 2 or MULT.
• The visa's period of validity begins before the first trip and ends after the second trip
• The visa's duration of stay is greater than or equal to the sum of the number of days you'll be in the Schengen area on both trips.

If the Italian consulate issues a visa that doesn't meet these requirements, you can apply for a second visa for your second trip with the Dutch, German, or Swiss consulate, as appropriate.

Do I need to submit flight tickets & hotel bookings for the second trip with my Italy application form? or only mentioning 2 entries is enough?

The more information you give them about the second trip, the more likely they are to grant a visa that will cover it. If I were you, I wouldn't count on it, however.

Is it better to issue 2 separate applications with single entry each? Or it is doable to have a 2 entries visa?

The main reason you'd want to avoid applying for one visa to cover both trips would be if such an application would increase the likelihood of a refusal for the first trip. I don't think it would, but I don't really know.