How do you do that, can you explain in more detail?
Go to custom functions menu 3, select customise controls
Set the * button (or whatever button you want to use) to "direct AF method selection".
This makes your * button cycle between AF modes (single point, subject detection, zone AF etc).
Now you need to disable the other focus modes. Go to AF menu 4 "limit AF selection" and turn off the other focus modes, but leave face tracking and single point modes on.
This essentially turns your * button into a toggle between subject and single point AF. I prefer this to specifying an AF-ON button as it lets me control where the camera focuses initially for eye AF (by setting "initial servo AF point for face tracking" to "initial AF pt set for face tracking") , and it means I can still use the AF-ON button for focusing.