Years ago I booked a TA solo because my sister didn’t know if she could arrange the time off. As a solo, I was paying for two, as the single supplement. When she said she could go, in just had to pay about $24 in taxes and fees.
Other times I book the two of us online, Early Saver, but there was no way to add her correct name. They always used my last name with TBA for her first name. I first tried to correct the name all at once, but ES looks at that as changing a person. So what I did was go into her data and modify, changing the first name, then saving. Then I went back into her reservation again to modify, and changed the last name. It worked.