I would like to reach that a video is slowly getting dark and another video then slowly brighter, while it is already played before it the full brightness reaches.
Please, excuses me for my bad English.
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What you want is a transition.
Open the Effect Palette and select Transitions. Then select Dissolve. I suggest using the Dip to Color transition. You might also want to try Motion Blend, Displacement Fade.
These are just some ideas. Best way to see if there's one that does what you want is to try them all. It might take half an hour.