What I would do is copy each end into a new document. For example...
1) Use the rectangular selection tool and draw around the left-hand edge you want.
2) Copy, make new image and paste. With keystrokes, that's Ctrl-C, Ctrl-N, Enter to accept the dimensions of what you just copied, and Ctrl-V.
3) Increase the width of the canvas with Modify > Canvas > Canvas Size.... so that you have room to add the right-hand edge.
4) Go back to the original image and use the rectangular selection tool and select the right-hand edge you want. Copy that.
5) Go to the new image and paste the copied selection. Adjust the positions of the two elements.
If you need to increase the canvas size, use Modify > Canvas > Canvas Size.... again. If your canvas is too wide, you can use Modify > Canvas > Trim Canvas (Ctrl-Alt-T).