With vanilla C2:
1. Select the bottom layer and set transparent to true.
2.Goto "Project Properties" and set "Clear Background" to "No".
3? If "Enable WebGL" is "on" add the brightness effect to the layout.
There you go, no clearing between frames.
For more control you could try out the canvas or paster plugins. Both work well when webgl is off, Paster is much faster with webgl on.