Well, my first thought is that if you have both with all of the layers then you can just go back and redo one. When both are done, save them as jpgs (as well as psds) and then create a new document that will fit both maps and then copy and paste them into the new document. You'll probably have a seam where the two maps meet in the ocean so use the Blur and Smudge tools to manipulate that seam. If you don't have them with full layers then combining them will require a lot of manipulation - blending, smudging, blurring, new pattern overlays and color overlays...in the end it would be easier to just redo one.