Can't say i've ever seen that if i'm reading your post right.
So your not applying any filters?
Could you post a screenshot?
If you can't see the difference in the 2-up Vertical arrangement - I'll take your word for it. It may just me. Thanks for your interest..
The original and copy at 25%:
Original and Copy.JPG
Using Arrange/2-up Vertical with both still at 25%
It would help if you posted a screenshot of what your seeing as someone may see what the problem is.
Also, what operating system are you using?
Thanks - my old eyesight. I put up another building and again saw what I thougt was a difference. I don't know PS well enough to have tried a blend/difference but I did, after you're message, manage to drag one document over the other and get a black screen. Thanks again, John