I'm trying to hide the borderline between the overlapping differently colored backgrounds. Has anybody a tip how to deal with this:
I have tried the brush tool, but the result wasn't acceptable.
Thanks and great day :)
Dodge Tool can be great for these cases. Select it, set it to use a soft edge brush and set Exposure to something as high as 60%, but leave Protect Tones. Use it to lighten the gray areas, being careful to not lighten the image itself.
If the gray is still visible disable Protect Tones and do your best to keep away from the models, as it'll lighten the colors very quickly.
Blur the border. Make a selection that covers the area where you want the smooth transition occur. Goto Filter > Blur > Motion blur, direction = 0 degrees. Put the preview on and move the slider for the wanted effect.
Gaussian Blur does not work here because it makes the top and bottom easily thin. You want the blur only horizontally.
Here's the selection, blur dialog and the preview of the result.
Another option is to remove the backgrounds totally and put a new that fits to the general design. It needs much work, but it's in wide use due it's great possiblities.
Assuming you have both these images on different layers, in my opinion, the best way to do this would be with a layer mask - since it's non-destructive. Non-destructive editing techniques are the way to go if you need to edit the image differently in the future, since the original image is unaffected.
Here I have set up a white background layer, with the images on two layers above. The top layer has a layer mask applied, which I edited with a large soft edged brush set to black.
Now, because it is masked out, you can easily drag the right side image over to the left, to get rid of the extra space.
Here's another approach based specifically on these images... This method may or may not be acceptable in your case
Hit Ctrl+C and then Ctrl+Shift+V to Paste in place
Go to Edit > Transform > Flip Horizontal
Right-click on a layer and select 'Merge Visible'
This gives us a transition which is not as harsh, and so gives us a little bit better starting point that we may come up with better results:
Now we'll perform essentially the same operation as user287001 has in their answer above, except instead of motion blur I used Box Blur:
And the final result: