How do I get a duplicate card for my wife who does not have a iPhone?

You can add her Apple ID as a participant. She does not need an iPhone for that only an iPad or Mac, however she will not be able to get her own physical Titanium card without an iPhone and she would only be able to use the card online and at Apple Stores.

click here ➜ Set up Apple Card Family and add a co-owner and participants - Apple Support

Basically, you don’t, without a lot of convoluted steps. The account is not designed to share, if the person doesn’t have an iPhone, iPad or Mac. In fact, its benefits are mediocre, at best, if you don’t have an Apple device.