XA requirements: The database adapter participates in distributed transactions and requires the back-end database setup for XA recovery by Oracle WebLogic Transaction Manager. Ensure that XA pre-requisites are met. For this, log on to SQL*Plus as a system user, for example, enter sqlplus "/ as sysdba" and then grant the following privileges:
Sql>Grant select on sys.dba_pending_transactions to public.
Sql>Grant execute on sys.dbms_xa to public.
Sql>Grant force any transaction to public;

In many cases you might have to pull data from different databases to show appropriate information in the UI. The crudest way is to use JDBC to fetch data from all the different databases.

If you want to ADFbc then you can have multiple AM's pointing to different databases. But if you want to use the datacontrols properly inside a taskflow (actually any page fragment as well), then you definitely have to understand the how ADFc layer creates the AM instances.

The following blog definitely helps you regarding all the information you need regarding taskflows and the how ADFbc goes hand in hand.

http://one-size-doesnt-fit-all.blogspot.com/2011/05/jdev-11g-task-flows-adf-bc-one-root.html