If you buy a new copy of QuarkXPress 8 before December 31, 2009, Quark will give you a second copy free. The best price I found was $755 at Software Media.
The process works like this: you buy QuarkXPress 8 from any Quark Authorized Reseller (or from Quark’s website at full retail price), and complete an online redemption process. You will then receive an email within 14 days with a serial number and validation code for your free license. Then you can install, activate and go!
Of course, you could get clever and start using your second copy right away — Quark has a 60-day free trial that would work on the second computer, and then you’d just permanently activate its license when you got your validation code.
QuarkXPress 8 is platform independent, so your two licenses could be on Mac OS X, Windows, or both.
Quark’s promotion has further details and the redemption form.