If your filenames are like this
e.g. 11111111111_a.txt or 11111111111_b.txt
LINQ will help you.
listBox1.DataSource = shortFilenames.Where(x => Path.GetFileNameWithoutExtension(x).EndsWith("a")).ToList();
listBox2.DataSource = shortFilenames.Where(x => Path.GetFileNameWithoutExtension(x).EndsWith("b")).ToList();
BTW It is recommended to assign more meaningful names to your user interface items.
e.g. listBox1 does not mean anything.
You could name the listBoxes e.g. listBoxLogA, listBoxLogB, listBoxLogC etc.