As jeff said if you just buy a single copy it will not work, you may be able to install and activate if both are not on at the same time but eventualy one will give out.
Buy an upgrade license coss, much cheaper and you can do a minty fresh install with it, microsoft just doesnt like you to know this.
You put the CD in the PC, boot from the CD, install it like your normaly would but dont put the CD Key in when prompted… When the install is finished and windows boots up you just activate it then and it works. Did it on my laptop at home which i was given an upgrade CD with.
For half the cost id get the upgrade