How can I set 2 Apple IDs on my iPhone 15?

There are several likely places where you can use a different Apple ID:

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

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

iPhones have 4 locations where an Apple ID can be logged in.

Settings ➜ Apple ID

Settings ➜ Apple ID ➜ Media & Purchases

Settings ➜ Messages

Settings ➜ Facetime

When you say "Apple ID for iPhone" what do you mean exactly? Which of the above are your referring to?

You can set a different Apple ID for iTunes (purchases and subscriptions) under Media & Purchases.

The Apple ID logged in directly under Settings ➜ Apple ID will then be used for iCloud only.

You can also change the Apple ID used for Messages and Facetime independently of each other and of the other 2 locations.