Actually coren2000 is correct. "And" means both "install" and "use" must be happening simultaneously for the license permissions to be exceeded. A violation would be one copy installed on two machines (or more) and being used simultaneously. Installation on more than one machine, as long as that software is not simultaneously in use on both, is not prohibited by the specific legal language of the license.@sanandra : I know my argument isn't very strong but it does exist (based on the opinion that it is unfair for a single user to be forced to buy multiple single user licences), if it ever came down to it I'd have to hire a really good lawyer... however OJ got off, Im sure I could find the right lawyer to get out of the contract if I was stubborn enough to make sure this went to court (which would cost way more than the $79 for the extra copy).
Legal issues aren't really of importance to the average joe however. (though they are to the business user who should really buy a business licence). What's of importance to the average joe is "am I being ethical when I install this software in two places?" and so long as you aren't sharing the software with anyone else I believe the answer to that is "yes."
Nowhere in the license does it say "it can exist on only one machine at a time". Nor does it say "install or use". Or would limit the license permissions to exclusive installation on only one machine. If the intention of the license was to limit it to installation on one machine that language would be very easy to formulate. It would simply say "install" without "and use". But "install and use" means I can install it on more than one machine as long as I am only using the software on one of those machines at any given moment. I can not let someone else use it on a 2nd machine at the same time that I am. That would be "install and use" on more than one machine at one time. Likewise, I am only one user, and no one else can use my private accounts on either of my computers, an iMac and a Macbook, so I am within the boundaries to have it installed in my account on both. It would be impossible or exceedingly difficult to be in front of both machines simultaneously using the single copy of the same software at the same time.
Edit: Oops. I re-read it. It does say it can't exist on two machines at once. So the second limitation exceeds the first and is a bit more specific, so install on two machines is violates the license. They should offer a two-machine single user license because calling it a "single user" defines the same "single user" on two machines in my case. It is misleading advertising right there on the box, the accepted definition of "user" being a "user account" and my "user account" is identical on both machines and I do not use them simultaneously. Take me to court? Go ahead, Apple. I'll go back to using Office.