A quick test yielded this:

State: HY000. Code: 10058. [NQODBC] [SQL_STATE: HY000] [nQSError: 10058] A general error has occurred. [nQSError: 22042] AGO function may not be used on 'Amount' because its source is horizontally partitioned. (HY000)
Checking metalink, I found document 739584.1 - AGO function with combined multiple sources. Enhancement request #7438154 has been raised and is targeted for 11g.
Cheers all!