An OEM Windows key will activate normally more than once (previous versions anyway, I have not tried with Windows 7).
As you do not seem to be too bothered by your scruples, even if it didn't work I'm sure you could ring up and lie convincingly enough to get the person on the other end of the phone to let you activate your second copy.
So in the short term it will work.
The problem is that every so often Windows gets in touch with Microsoft. As I haven't tried it, I don't know whether Nicrosoft's update thingy will be bothered about two computers using the same key.
I believe that Volume licence keys that appear on the internet soon stop working as they get blocked, but does Microsoft really trawl through the logs of all the machines that connect to check whether a key is used on two machines?
Basically it comes down to how much of a gambler you are. If Microsoft don't block then you have got away with saving a few quid (or whatever), but if your key does get blocked, it won't work on either machine, so you will be forking out for two more licences as well as having a rather pissed off girlfriend.