I Think the idea of panter can work if we modify it:
What about leaving off the signal of the fast track?
The slow train reserves the path to its signal, the following fast train is waiting at a previous signal until the slow train enters the bottom track. Then the fast train will reserverve a possible route to the next signal/station behind the double track area and the slow train waits at its signal.
I haven’t tried it out but I will do.
If you want I can post a picture to illustrate what I wanted do explain.