To mix between two audio sources:

1	Choose New Control Window from the File menu
2	If only one control window is open, choose it again (at least two control windows must be open)
3	Drag a file onto one control window to begin playing
4	Drag a file onto another control window to begin playing
5	Choose Mixer from the Tools menu
6	Select your two sources with the pop-up menus
7	Slide the mixing slider to crossfade and mix!

It's also possible to set up two control windows to play from the same active playlist, and mix between them.