Hi people,
I travelled yesterday NCE_LHR-BOS, ticketed AA but first leg on BA. I am OW Sapphire. BA let me check-in for free two bags in Nice (woot-woot). After a couple of phone calls, she confirmed that if the ticket is on AA and the TATL is on AA as well, I am entitled to check two bags for free on BA as well!

Very happy to have saved 40 GBP