How do I make two iPhones share the exact same data and iCloud account?

If both phones belong to yourself, then

• Using the same Apple ID on both
• Setting up both phones to use iCloud synchronization (Set up iCloud on all your devices - Apple Support)
• Setting up both phones to use the same e-mail accounts (and associated data like calendars)

will go a long way.

There still may be data that potentially would not be synchronized, depending on how your third-party apps are designed to store data. But some apps might use iCloud Drive behind the scenes – or be designed so that you would be able to synchronize data through another cloud service. E.g., if you subscribed to Microsoft 365, and were using the mobile Word + Excel + PowerPoint + OneDrive apps, I'm guessing that Microsoft would have set things up to make it easy to store documents in (and synchronize them via) OneDrive (Microsoft's cloud drive).

you just setup one using the other

Use Quick Start to transfer data to a new iPhone or iPad - Apple Support

same as one often would if setting up an ipad for that matter, and setting up a new iphone before selling or recycling the old one

you should only do it for iphone you yourself should use, if you want somebody else to use one of them you're in a world of hurt, they get all the same imessages, share call log and the likes. appleIDs should never ever ever be shared

Make sure each family member has a unique Apple Account – Apple Support (UK)