Is there just one cash account to be used on both Treasury and the agencies books?
Yes. The same account number will be used because of its used by different business units. The same Agency Cash Account (101005) will be used between OTFS and all Streamlined Banking agencies. This will be the main cash account, but if you have a local depository account, the local cash accounts will still be used (e.g. 101060). Business Units is how you differentiate the activity in the 101005 account.