I find it odd you cant get it to work.
I did a very simple test on this.
I started a new game, I set up 2 stations, with a single line between each.
I then set up signals and described, and set up 2 trains to run on that same line.
It worked just fine, no problems at all.
Are you sure you set up the signals correctly?
You didnt somehow place the 2 inner signals by the same track?
It requires some precision placement of the signals, so zooming in close will help.