From AA's site:
Standard checked bag fees of your destination apply up to 50 lbs / 23 kgs and 126 in / 320 cm (length + width + height) or $150 fee applies from 51 lbs / 23 kgs to 70 lbs / 32 kgs.
Seperately on their site, the list prices for 51-70 lb bags ($100 ea) and 71-100lb bags (
https://www.aa.com/i18n/travel-info/...ht-baggage.jsp)
The size limit is generous. A typical tandem-sized cardboard box, Bike Pro tandem case, or Scicon tandem case all fit within the 126in size limit. The weigh limit will be much more of a problem. IIRC, the Bike Pro case is 45lb empty, so that weight limit comes quickly. The other full-size cases weigh around 32lb, so it should be possible to at least pack the frameset into one. Wheels, pedals, seatposts, tools will have to go in another box/bag/case to stay under that 50lb mark.
The real winners here are the tandem riders who can fit their frame into a single bike case (Thule Roundtrip, Evoc XL, etc) and pack their wheels/tools in a second bag to stay under the weight limit. Those of us who use 2 S&S cases will continue to not pay bike fees.