Indeed it's BA only (and in some contract lounges you won't get 2 guests either). But in the case of AA, you can simply buy a day pass for the second guest. If you chat up the AA agent and then ask to pay for the second guest, she may well just wave you in, though you can't take that for granted. You are aware that Admiral lounges aren't so impressive (apart perhaps from SFO, ORD and JFK)? In many airports there would be better places to go.