I actually did just this over the phone the other day.

Not possible via the website, but the BA holidays team managed no problem.

They initially price up for two people, then remove one of the flight bookings. The reason I needed to do this was because the hotel I was interested in only offered their “solo” rooms when booking as a single traveller on BAH; however the hotel website showed lots of other room categories, which then became available over the phone once “two” people were in the room