On windows I don't I ever saw steam download more than 1 game at once. All are paused except for one. Maybe there's a bug here that is causing the download to fail instead of just pausing, but i think it's a "by design" decision for steam to not download more than 1 game at once.
@mewt666 I was always able to download multiple games on Windows. When I buy Humble Bundles in particular, I download every game at once overnight so I can play them in the morning. I always checked to make sure they were all downloading, and sure enough, every game was downloading together.
My experience is that on Windows, it used to download in parallel, but since a recent update the client only downloads one game at a time. I'm guessing this is done on purpose to limit the number of concurrent connections to valve servers.
GCF and NCF games could download simultaneously, but the newer ACF format will only ever download one at a time. All NCF games are slowly being transitioned to ACF (this is now largely complete - at least for the 250+ games I own), which is why "recently" you very rarely see multiple games downloading at the same time.
The change to just a single active download is an intentional change, so I'm closing this.
Nothing extracted yet.
When trying to download more than one game at once (Cogs and Frozen Synapse in my case), Steam will fail to download them both. As soon as the second game is selected to download, the first pauses. If you unpause it so that both should be downloading at the same time, neither game will download. It will just sit there, acting like they're paused, when they're not.
System info: