The idea of a data manager is that it...manages data. It takes a request, such as building a forum, and updates all the tables and fields and whatever so that the update takes place. Note for instance that the vb forum datamanager does something to update the datastore (I don't know what, I have never looked).
In summary then your one data manager should manage all the data.
If the secondary table is fixed, just hard wire it into your code.
PHP Code:
/**
* The main table this class deals with
*
* @var string
*/
var $table = 'scst_fb';
/**
* and here is my other table
* @var string
*/
var $otherttable = 'other table';
Then just build up your code to do whatever it is that needs to be done. You can always add extra setters to include subsidiary data if required.