How to add 2 Apple ID's to one iPhone?

You can only do that in very limited ways. iPhones are designed to only have one primary user. You usually only change accounts if you are truly no longer using the device long-term such as giving it away or selling it.

Here are places where you can use a different Apple ID:

1. In Mail if you want to use a different, or more than one account. Settings -> mail -> accounts -> add account

2. Messages and Facetime. Sign into a different Apple ID for these services in Settings. Note this has to be the same ID for both. Go to Settings > Messages > Send & Receive, and make sure both your telephone number and Apple ID are input there and both show a checkmark. If you need more help, check this link: Add or remove your phone number in Messages or FaceTime - Apple Support .

3. In the support article: If you subscribe to Apple One and use a different Apple ID for iCloud - Apple Support Apple alludes to using a different Apple ID just for media, subscriptions, and purchases, but doesn't describe how to do this. See if these work:

In iOS/iPadOS: Go to Settings > [your Apple ID] > Media & Purchases. Sign out of the current ID and sign in with a different one.

In Ventura or later: First, quit Settings. Then, in App Store, choose “Sign Out” in the Store menu, then click on the “sign in” button that appears in the main window. You can then sign into the other account. (from: Using different Apple ID for media & … - Apple Community )

4. Use a different Apple ID to share purchases with Family Sharing as outlined in Use a different Apple ID to share purchases with Family Sharing - Apple Support