If there are 2 or more developers who have developed the App for AppStore, How can they have both of their names displayed below the app together?
Also, do they all need to have a developer account or one is enough?
You need a single paid developer agreement.
Only the legal name of the entity (person or company) that entered into the developer distribution agreement with Apple will be shown on the App Store.
You can have whatever credits you want inside the app