Feruniromantic and Ferunisexual (terms coined by me)
Feruniromantic: someone who only feels strong romantic attraction to one person and feels romantic attraction to one or more other people, but the romantic attraction felt for them is so weak it may as well be nonexistent.
Ferunisexual: someone who only feels strong sexual attraction to one person and feels sexual attraction to one or more other people, but the sexual attraction felt for them is so weak it may as well be nonexistent.
These are similar to uniromantic, which means someone who only feels romantic attraction to one person, and unisexual, which means someone who only feels sexual attraction to one person.
"Fer-" comes from the Latin word "fere" which means almost, nearly, about, generally, usually, or quite.