Conditional Logic

You can add field B to your form and make it required. Hide it with conditional logic that says "if field A is selected, then show this field." If field A is selected, field B will show and be required. If field A is not selected, field B will never show, and will still pass validation even though it is hidden and required.

Let me know if I missed something or overlooked an obvious shortcoming in this approach. I use a similar approach here: http://gravity.chrishajer.com/contact-us-left-and-right/

You would not need two choices for A (as I use radio buttons here for call or email.) However, the phone and email fields are both required, and only one will even be submitted, because they are both hidden with conditional logic and only one will ever show.