If you want to do something like that you should create a track like this:
then assign signal 1 as waypoint for slower freight train and signal 2 as waypoint for faster express train. I have no time at the moment to test how well it will work but it should work like what you want to do. The frequency of signals before the loop should be higher as it is on diagram to reduce distance between freight and express trains. The length of the side track should be at last twice as long as the freight train and there is also a possibility that you will have to add more signals to side track to prevent freight train from reserving the junction where the tracks merge again.