Last time I had this scenario I called BA and went through the booking process over the telephone after confirming availability. My wife was present to confirm her account details and provide permission for using the 241 from her account. No off line fee was charged. You can also make 2 separate bookings but I decided to call and explain my position. The agent was extremely helpful. Granted it was 5 years ago but I don't think much has changed since then. I didn't go down the 2 browser route as someone suggests in the thread below.

Thread from last year may help you further.

http://www.flyertalk.com/forum/briti...my-wife-s.html