why cannot view both outline and mind-map at the same time?

Well... I believe the outline view is initially provided as a gesture to users who may not know the mindmap benefits. But it is available for you at least.
If you have the luxury of more screens, you could try to have the map open in Map view. Find the small "detach Tab" function in the tabs.Tear it off and drag it to your second screen. And if you then open the same map again but now in outline view, you may have what you want.

Another option MindManager provides is the use of the MAP index view.It is in the TaskPane tabs at the rigt of your screen and you have marker index & element index views. They are really helpful for those who like to combine list view with map views