As Lisa said
File> New and enter 12" x 24 inch as your specs (or whatever the size you scrap just double your width)
Something that may help>
Choose Insert Line, rotate the line so it is vertical and the choose Format> Resize Object to Fit>Scale to Fit and the line will divide the page in half exactly
When you have finished your layout, save and then Edit> Duplicate whole project
Edit> Flatten all Objects (except the dividing line)and then use the Marquee tool to select one half and paste on a new canvas to print and same for teh other side
HTH