How do I separate my phone from my husband's?
My husband inherited my old iPhone. Now, some (but not all) of his texts appear on my phone and vice-versa. Annoying for both of us. What's the fix?
You need to erase the phone that you gave him, have him set up his own Apple ID and start over. If he has his own Apple ID, you should still erase the old phone and let him start over with his ID.
This what you should do with your old phone.
What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support
If you want to share purchased content, set up Family Sharing.
Set up Family Sharing - Apple Support