iPad data compatibilty with iPhone Duo

If you mean transferring your iPad data to a new iPhone Duo during setup, yes. You can use Quick Start with an iPad to set up a new iPhone and transfer compatible apps and data. Use Quick Start to transfer data to a new iPhone or iPad - Apple Support

It isn’t really a “merge” of two devices, though. If the iPhone Duo is already set up and contains data you want to keep, I’d use the same Apple Account and iCloud to sync things such as Photos, Contacts, Notes, and Messages rather than erase it and transfer again.

Some iPad-specific apps, settings, and data may not carry over to the iPhone.

Yes — one important example would be anything stored only on the iPad rather than in iCloud.

If I were replacing both devices, I’d set up the Duo from the iPhone 17, then let iCloud bring over things you already sync, such as Photos, Contacts, Notes, Calendars, and iCloud Drive.

Before getting rid of the iPad, I’d specifically check Files > On My iPad and any iPad-only apps you use. Local files or app projects may need to be moved to iCloud Drive or exported first. iPad-specific things such as its Home Screen layout, multitasking settings, and settings for apps that don’t have an iPhone version won’t really have anything to transfer to.

So I wouldn’t erase or trade in the iPad until you’ve confirmed that anything important from it is accessible on the Duo or safely backed up elsewhere.