From the Manual

If you are using a printing service that does not work with full double-wide layouts, in this video I'll show you how to cut your pages in half and save them individually. 

Save Side A

• Open a double-page JPEG file (File > Open) from following the last video.
• Zoom out (Ctrl/Cmd -) to see the edges of the image.
• Get the Rectangular Marquee tool.
• In the Tool Options click on the New Selection icon. The Feather should be 0 and the Aspect should be Normal. (PS: The Style should be normal.)
• On the document, click at the top left corner outside of the image boundaries and drag down until the selection outline fills the left side of the image.
• In the Menu bar choose Image > Crop.
• Choose File > Save As.
• In the dialog box, navigate to the same folder that contains the original image.
• In Save As, click right after the number and put an A for side one. Choose JPEG for the Format and click Save.
• In the dialog box, set the Quality to 12 and the Format to Baseline. Click OK.

Save Side B

• Press Ctrl Z (Mac: Cmd Z) to undo the crop.
• On the document, click and drag outside the right corner of the image down until the whole right side of the image is selected.
• Repeat the save process above except rename with a B.
• Close the document without saving.

Repeat for each double-sided JPEG file. 

From the Transcript

If you are using a printing service that does not work with full double-wide layouts, in this video I'll show you how to cut your pages in half and save them individually. 

Begin by opening one of your JPEG files. You should still able to see the guide in the very center of the image, but not to worry the guide will not print out on your page. Next you'll want to zoom out so you can clearly see the edges of the image. To do that hold down the Ctrl or Cmd key and push the - key. Next, get the Rectangular Marquee tool. In the Tool Options click on the New Selection icon. The Feather should be 0 and the Aspect or the Style should be Normal. On the image click at the top left corner outside of the image boundaries and drag down until the selection outline fills the left side of the image. The selection should naturally snap to the guide. Then in the Menu bar choose Image > Crop. 

Then choose File > Save As. Navigate to the same folder that contains the original image and then in the Save As box, click right after the number and put an A for side one or a B for side two. Again, it'll be JPEG. I'll click Save and the Quality should be 12, Baseline should be the Format, and click OK. Now I'm going to press Ctrl Z, Cmd Z on a Mac, to undo that crop. 

This time I'm going to click and drag outside the right corner of the image down until the whole right side of the image is selected. And then I'll repeat the process. Choose Image > Crop and then File > Save As. This time I'm going to save the file as a B for the right side. Again its going to be JPEG. And then I'll click Save. And one more time, the Quality will be 12 and the Format will be Baseline. Then click OK.

Then to finish up go ahead and close this file and open the next one. When you're finished your folder of images should have the full length version and then an A an a B version which coordinates with the left side of the image and the right side of the image. Now you are ready to upload all these individual pages to a photo print. So, that's how you cut your JPEG files in half and prepare them for printing!