This can be done pretty easily with a database type form.
Create a new form and select database in the wizard.
Select the customer table and add the location table as a child.
This will build a form which can query the records as you desire.
Play with the default form so you understand the process.
Than add your secondary DC of the IDs and then use your dropdown to have a rule so that the ID chosen sets the query field and executes the query.
The problem with these types of forms are:
They must be built from the start as a DB type.
You can only query and submit the one table and its children
All children are created when a parent is inserted
no large file types
no browser form compatibility
A better solution is to use web services, which improves performance, gives you all of the flexibilty and removes the limitations I listed. Check out www.qdabra.com for more info.