To confirm we are 4 persons traveling F on BA LHR-MIA-LHR using 2 Amex 2-4-1's

I'm looking to add AA TPA-MIA to connect with our return MIA-LHR flight. BA have advised that the additional internal flight can be "linked" to our exisiting booking but it will be under a different reference. BA cannot confirm if our BA F baggage allowance can be used on the AA flight. Can anyone advise what the baggage allowance would be on the internal flight under the above circumstance ^