The problem was, that strangely I did pass the same image, here the corrected Code:
private unsafe static void combine_img(Bitmap img1, Bitmap img2)
{
BitmapData image1 = img1.LockBits(new Rectangle(0, 0, img1.Width, img1.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
BitmapData image2 = img2.LockBits(new Rectangle(0, 0, img2.Width, img2.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
int bytesPerPixel = 3;
byte* scan1 = (byte*)image1.Scan0.ToPointer();
int stride1 = image1.Stride;
byte* scan2 = (byte*)image2.Scan0.ToPointer();
int stride2 = image2.Stride;
for (int y = 0; y < img1.Height; y++)
{
byte* row1 = scan1 + (y * stride1);
byte* row2 = scan2 + (y * stride2);
for (int x = 0; x < img1.Width; x++)
{
if (row2[x * bytesPerPixel] == 255)
row1[x * bytesPerPixel] = row1[x * bytesPerPixel - 1] = row1[x * bytesPerPixel-2] = 255;
}
}
img1.UnlockBits(image1);
img2.UnlockBits(image2);
}