Unable to add husband's iPhone to AppleCare One plan

I purchased AppleCare One coverage thinking I could add my new phone and my husbands new phone (both purchased at the same time direct from Apple) but I can add my device and my watch but my husbands phone isn’t listed on mine and when he tries to add it it wants him to pay the $19.99 , not add him to my plan. How do I get his on mine? I wouldn’t have purchased that one if I couldn’t put both on.