This produce a wxSplitterWindow and two associated panels as well. You can choice vertical or horizontal splitting.

In the properties window you can choice the styles and the sash position.

Be aware to do not put in a panel under a splitter, a widget too long or big, because in the design window you cannot notice nothing abnormal, but when you run your program, one of two panels will takes all the avalaible space and the other panel will shrink to the minimum size possible.