I booked a flight with Frontier about a month ago for early October, and Flight Flexibility was included for free due to COVID, I presume.

I booked the roundtrip flight with only me as the passenger. My question is, if I decide to change the flight and add another passenger for a one way flight, the new cost for both of us still being less than the original flight cost, can I apply it for both passengers? Or will it only apply to my own flight, with me having to pay for the other passenger, thus losing part of my credit?

Although I've flown with Frontier for several years now, this is the first time I will be using Flight Flexibility. The website is not very helpful regarding about the aforementioned situation.