Why am I having to install and patch the game a second time to let my girlfriend play

[Indra Echo]

All I can think is its account tied on their server.

It seems likely that it's something like this. The patches are resident on your machine, but some part of them is tied to the server-and some content is integrated within the patches like missions and all that and is tied to each gamertag and the character data.

[Nerotic]

Must be a pretty slick job sitting back juggling 3 platforms (two being consoles with their own architexture) and a link in with the TV show.
I'm sure a puppy, a kitten and a baby could do better. Right? Right?

Not saying they got an easy job.
They took a big ambitious bite but don't seem like they have enough time to chew before they swallow.

[JxSiN]

Like previously stated, it is because of the separate accounts. The account is what is linked to their servers, not your game. All the new account info has to be saved, but any downloads that match (like certain info) is pretty much just overwritten. You will have to use more of your hard drive, but not necessarily double.

And before people go off saying that Trion should have done it differently, go check any game where it saves to the company's servers. DCUO is a great example, as you basically have to re-download every patch and item (but what you will notice is that it is faster the second time around.... cuz you already have some of the info it is trying to download).