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IDK about anybody else, but I need clarification! What really are you trying to do?

Here's where you lose me: " by resizing the whole image by the width of line." I think you mean you don't want to lose any part of the image, so use your crop tool to extend the canvas till you include the entire original image after making your changes with the line and such.

Anyway, if you are wanting to move top and bottom parts of image apart basically and have a line show between, it is a 2-step process if you do it with the marquee tool. 

Bottom layer, fill with color or pattern, or make layer above image to add your line. Image layer, use rectangular marquis to select half of image, change tool to move tool, arrow your half image into place. If you're satisfied, third step, merge the layers. Doesn't seem like much in the way of concatenation to me. But maybe this isn't what you mean.