I've solved my problem by myself:
It's about the given
My screen has the resolution of 1920 x 1080.
In VS the TargetResolution is half as big as the real resolution: 960 x 540.
To make sure my calculation is correct, i have to get the real resolution. Thats why I wrote
res.Y * 2
res.X * 2
While running the game in VS, it works, but the TargetResolution will be the real resolution, if I run this in the Dualitor. In consequence of that the scaling of the image is twice as big as it should be.
I've fixed the code for the duality launcher (without * 2), because I won't play my game in VS, if it's finished.
I hope you can ignore my bad grammar and understand my solution nevertheless