The Combined View in the Gamma Window displays the curves of all the active layers together in one window. This lets you to make exactly the same adjustments to all of the curves at the same time. You simply need to switch on the option highlighted in the image below to display the Combined View. 

It is very useful if you are editing the curves for many layers and you want to reduce the size of the Gamma Window on your screen, because you can resize it to show only this window. It can be helpful to adjust the individual curves in their own panes, but also see how they compare together in the combined view.

To reduce the number of panes visible in the Gamma Window you can switch off the individual panes by clicking on the button shown below.