Over 50% of the nodes have to be compromised in order to double spend, or change the history of the shared ledger.
So assuming all of the banks have the same number of nodes/the same computing power, it would take >50% collusion to cheat the system.
Well if each block has to be signed with 50% of the banks that reduces trust by some degree, however this still can be done more efficiently with a relational database.
That's what they have now. A third party clearing house (presumably using a relational database) that is the central authority on all intermediary transactions. Creating a blockchain is about removing that third party and instead breaking the trust up between the participating banks.