This post is extremely old, but still relevant to my experience so I don't think I should start a new thread. I'm having the same trouble, it really kills my efficiency in culling images.
Say I'm flicking through images in preview mode, rating, tagging etc. I want to compare two images from the same burst of shots for sharpness to cull one.
First time I do this in a session by hitting V, it will open the side by side windows. The image I had selected will be in one, and the other will be blank, and I can click that and then click the image next to my selection that I want to compare. That's fine. I tag the one I like and go back to full view with O and continue.
But then, say a few dozen photos later, I get up to another photo I want to compare to one next to it from another burst of shots. Well this time I hit V - the one I had selected will of course be on one side, but the other side contains the photo from ages ago that I don't need any more. The problem is when I click the half I want to change, the bottom strip of images will jump back a few dozen photos to the left, and then I have to fiddle around with scrolling back to the right to try and find the photo that was right next to the one I originally had selected, which is often slow, confusing and disorientating with many similar shots.
Is there no simple way to tell PM which two I want to compare?