I am using one Apple ID to my two Iphones. How do I assign a different apple ID to my other phone.

The simplest way is to visit https://appleid.apple.com/account#!&page=create and create your "second" Apple ID. Then, on the phone that has fewer paid-for applications and subscriptions, go to Settings | Apple ID. Turn off "Find My" and sign out of your old, shared Apple ID. Then sign into your newly created Apple ID and enjoy the independence of your two phones.

See How to create a new Apple ID https://support.apple.com/en-us/HT204316  and

How to switch Apple IDs on your iPhone or iPad

https://www.imore.com/how-change-apple-id-your-iphone-or-ipad

See the data implications below.

Some of these things may apply even though not changing an Apple ID: What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

 Keep in mind that some things available under a first Apple ID will not be available under a new Apple ID. For example, you won’t be able to transfer anything purchased from the App and iTunes Stores to the new ID. And, there are other issues to be addressed.

And see what Eric Root says on this matter here  https://discussions.apple.com/thread/251483565 and

what Winston Churchill says about it here: https://discussions.apple.com/thread/7452747

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But, despite any other data loss you can set up family sharing between the IDs to share purchases made with them. See:

Family Sharing  https://support.apple.com/en-us/HT201060

Set up Family Sharing https://support.apple.com/en-us/HT201088