It's perfectly OK to use the same single-user license on more than one of your own computers (your desktop and laptop for instance). You just need to request your license key once on each machine. This also applies for those that have more than one Mac, but boot from an external drive that contains your home folder.
Just startup (from the external drive) the Mac you want to license Spell Catcher on, then request your key. Repeat for each Mac. Don't be phased by the fact that you will end up with more than one license appearing in the License Manager window, and that all but one will be displayed as "Not for this installation". As long as there is one license in the list with the blue arrow, and the caption at the top of the License Manager window says "Software Licensed.", you're all set.
Remember - don't type in your code, name or e-mail! Just drag and drop (or copy and paste) the three lines containing your name, e-mail and license code from the e-mail to the License Manager window. These three lines appear in the e-mail you received when you purchased Spell Catcher X, or the one you get from our Lost License Codes form, or from the file with your licensing instructions (select an active license in the list, choose "Save As..." from the File menu.