I had a massiv traffic jam at my airport, due to a bug (37334) where airplanes shuffle the position in the row before a runway, wich leads to long time period where no plane starts. So I disabled GA flights, allowed to use both runways, and counted the flights on in game day long.
The result:
Left runway 136 starts 88%
Right runway 19 starts 12%
I have no clue when and why the airplanes decide to choose the runway but at the moment I think if you have more traffic than one runway can handle you’ll get big problems, in my case airline rating sunk, no more flights offers, less traffic, problem solved 
It might be useful if you could set a decision point in my case at the t-junction where the planes check wich runway they use.