Once you've cropped that side you can save it as a jpg or a new psd (with a different name).
This tutorial is for those of you who would like to take a 24x12 page and adjust it to make 2 - 12x12 pages. I'm going to use one of my State Templates as an example, I'm also using Photoshop CS3. First what you will do is make your page like you normally would, add your photos, papers, embellishments etc...and save it as a psd, under a new name so you don't edit the original template(If you need more help with this I have a tutorial here.) My example isn't exactly finished I'm just using the plain psd as an example (you can pretend all the pretty papers, pictures and embellishments are there). Then with my Rectangular Marquee Tool selected I will go select fixed size and enter 12in by 12 in. Shown below:After that is done, you will click on the left edge of the psd and it will select the left side of the page (exactly 12x12). Then go to image crop.
Now once you have saved that side you can go back to your full page 24x12 finished page and do the same thing to the right side of the page (save it with a different name) and you have two 12x12 pages that will fit together perfectly. Hope this tutorial is helpful.