You can't (Unless you do unrecommended workarounds and set the split with script that run at startup)
Because MC is designed around the dual panel layout where one panel is default source and the "other" panel is the default target. It is not designed to be a single panel filemanager like Windows Explorer..
Even if you hide the panel (F11) it is still there and many function in MC depends on it and by hiding it you might do things in the panel that you don't know is there. So to avoid you messing up files, it will now save a 100/0 panel split.