Sorry, one retail license (full or upgrade but not oem) is only good for one single install (at a time). You need to obtain a second separate key for the second install. I use MS Technet (normally $350 per single year paid subscription, but frequently there are $100 discounts available) which gives access to most, if not all MS software and in regards to OS's, gives 10 unique retail keys that don't ever expire for each version of windows 7 (and all the other MS OS's).
The technet license is technically "for evaluation only" but MS seems to let home users slide and has a very generous definition of evaluation in actual practice. So many people have reportedly been using Technet OS and application keys at home for years. What MS doesn't like or permit is using the Technet keys in what they call a "production environment" (i.e., business) but "home" use seems to be completely allowed. (Of course selling or giving away one's technet keys is a gross violation of the technet license so please don't anyone PM me about that as such PM's will be ignored.)
Obviously, it's hard to distinguish between home and business in many cases and I don't think anyone at MS is paying much attention to such fine points. But that's just my opinion.
Anyway, if all you need is one more key, it's cheaper to just buy it at retail or online. But think about the fact that Office 2010 final is supposed to be released within a year and that will also be available to current active Technet subscribers at the time Office 2010 is released or likely even before general availability as was the case with Windows 7. So Technet at times like this can be a very good deal for the right situations.