Change it to 2 adults then - unless that makes the price go up?
The only way to book from your account but have the lead guest name be someone else is on the phone (and agents who aren't based in the US seem to be unable to do it in my experience)
I thought I saw a higher price when I booked yesterday, but I must have been dreaming. I edited to two people for no additional points, with her listed as an additional guest. The system says an additional guest can check in, so it should be fine.
A bit bizarre that the system had originally let me reserve for myself and an additional guest as one adult.