This video shows that you can actually move the mixing circle (yellow) inside the mixing canvas and over the sources instead of moving the sources themselves.
Take the case where you have two groups of videos, that you can move in and out of the mixing circle to make this 'composition' more or less visible. To fade from one group to another, you could create a group with all the sources, but you would loose the flexibility of moving them individually.
The solution is to grab the mixing circle by its border (becoming lighter when the mouse is over) and to drag it along using the middle mouse button (like for navigating into the view, but with the mouse over the circle border). By moving the circle, you also alter the visibility of the sources, and you can choose which source to target by centering the circle over it.
NB: If you don't have a middle mouse button, specify how you want to enable the navigate and the drag mouse actions in the preferences ( File / Preferences / Interface / Mouse (or stilius) buttons)