You search for special seats utilizing miles. They are BA miles - you speak to BA. You can do it online via your BAEC page, however the partner flight availability is a little hit and miss online - you may have to call to see exactly what is available.

BA does have a family ("household") account to combine miles, however you are better off buying one way tickets than a return anyway. If your wife buys two tickets one way and you buy two tickets on the way back, you'll be on the same PNR and won't have to bother with the family account.