Better shown through example, here is the desired behaviour:
~/documents/doc1.txt, ~/documents/doc2.txt, and ~/downloads exist.
The user types:
Because this completion is not ambiguous (there are no other completions that start with doc), it is expanded to ~/documents/ however in ADDITION, another tab-completion is done automatically afterwards, so after the first tab the user sees:
vim ~/documents/ doc1.txt doc2.txt
Essentially where the user would normally have to press tab twice here, now they only need to press it once.