It seems that the only way to acquire Half-Life 2: Episode Two will be to purchase the entirety of Half-Life 2 (+ expansions) again, and use only the one product code. This seems like a really absurd idea…and I cannot imagine why it was difficult to get just the expansion and extra content into a nice retail package. But someone deemed it unworthy.

I like how they describe what you should do in the official statement:

_Those who purchase the PC version of The Orange Box will get three separate Steam product codes: One for the three new games, one for HL2, and one for Episode One. This way a PC Orange Box owner can give away their Half-Life 2 or Episode One unused Steam product codes if they don’t need a copy of those games. _

Ah. So, you buy the game, and if you already have Half-Life 2 and Episode One, then you have to barter with your friends to find a buyer. That’s just special.