If the license says it can be installed on more than one computer as long as it is only in use on one at a time, then there is no legal problem with what you are proposing. I don’t think I have ever seen a license agreement like that, though.
My example of paying full price for one game and only paying cost for the other is good, but maybe I should add you pay the full cost of manufacturing and stocking the game. It would still be unethical. You are depriving the seller of the profit they would otherwise gain. No, they did not lose anything they had already, they lost their potential sales.
Time and again games that are difficult to pirate sell unusually high numbers of copies. This is because an estimated 50% of games out there are pirated copies. One of the reasons StarCraft is STILL one of the top selling games years after it’s realease is that if you want to play it with your friends over Battle.net you have to have the key code off of the CD case. When the game first came out the biggest ‘bug-report’ was that people were taking the game home, starting it up, and finding they weren’t allowed to play online. It turned out that it was because people were buying the game, making a copy, and registering on Battle.net with the code on the case, then returning the game to the store - somebody later buys the re-shrinkwrapped game and couldn’t enjoy it to it’s fullest because someone had stolen the key-code they bought the rights to. When they took care of this, all the people who got hooked on their pirated games had to go buy it again to play.
Piracy is stealing, plain and simple. You may feel more justified stealing from people you will never see face to face, but that doesn’t make it right.