It seems to me that the hold at authorisation was for the amount of two tickets, say £500, but the transaction presented for settlement (payment) was for two separate transactions of say £250. Whilst this is normal practice for BA ticketing and payments, something has gone wrong, probably with BA not releasing their authorisation once they presented for settlement, but it is possible that the bank might have missed the release instruction as the settlement was across two transactions verses the one larger amount for authorisation.

But for the OP to work out who is responsible between BA, their aquiring bank, the card brand, and the OPs card issuing bank is hard as it's all very opaque.

The only positive news is that hold/authorisation will usually time out, but it does usually take two weeks.