Minster of Finance Bos has outlined in a reply to MP-questions that the supervisors of ABN AMRO are now working out a Memorandum of Understanding to determine how to supervise the entities of Barclays/ABN AMRO. See also IEX.
The answers to the questions actually look a bit like the answers to a student exam question. It is a precise and formal formulation of the rules applicable. DNB will remain doing DNB-local Dutch supervision, as will Barclays. But in the case of disputes, the FSA will be the boss. The answers also specify that DNB has a more hands-on supervision approach; it more often visits the banks it supervises.