I wanted to get rid off one data source in tableau, that's why instead of using 2 different data source for one dashboard, I wanted to copy all relevant fields from one data source to other. Is there any way in tableau , by which I can copy-paste a field from one to other data source ?
Unfortunately, there's no straightforward way to do this.
Fields that you are referring to look like actual columns in the database (?) behind the published datasource. In order to replicate these you'd have to modify and republish this datasource.