It seems that the documentation for AddCtrlToSplitter() function is incomplete and/or partially worng.
This function attaches a new control to the splitter control.
First of all, it's not clear if parmeter controlID refers to the splitter or to the control to attach.
From my tests it seem that this is the splitter control.
Second point: parameter controlToAttach has no description.
Based on its name, it seems that this is the control to attach 
Could someone from NI side clarify and improve the documentation?