The design of Graphic User Interface (GUI) will depend on the design of the simulation and what is the effect you want to show.
There is no fixed answer. It is all depends on what kind of effect you want to present.
I did not use split panel very often.
Normally, I want to have the GUI as simple as possible. (In the past, I want to have as many features in on simulation as possible. But later I found it is not easy for students to understand all the meaning of different GUI components.)
You can upload you source and let me know what you want. May be I can give you some suggestion. ;)