Suppose you're running multiple WSN scripts (or multiple installations of the same script). You want to let people login with their username/email at script A, but if there's no such username/email in the members table of script A you'd like to check script B's members table.

In your tweaks.php, add a line
$fallbackmembers = 'otherscript_members';

That's all there is to it.

This feature doesn't support non-WSN scripts at the moment, be sure to express your interest if you'd like integration files to work as fallbacks.