Portal 2 Co-op with a single Steam account?

I picked up Portal 2 and wanted to try co-op with my son. We have two computers, but just one Steam account. Usually this is fine since I just leave the family computer in offline mode. Unfortunately it would appear that Portal 2 only does co-op when Steam is online.

Is there a way to do Portal 2 Co-op / multiplayer with a single Steam account on two PC's? I saw you can with a PS3, and there is no split screen on a single PC.

As it sounds like you're aware, you cannot be logged in to Steam in two separate locations at the same time (unless one or both are in offline mode, and thus unable to play online).

But, as badp notes, another question talks about a hack for single-PC splitscreen.

Edit: Suprisingly, it looks like you can play with Steam offline over a LAN:

Go to "C:\Program Files (x86)\Steam\steamapps\common\portal 2\portal2\scripts" or to the relevant folder or drive where you installed Portal 2. Now you have to add the following code in "extras.txt":

"lan"
{
    "title"     "Host LAN"
    "subtitle"  "sv_lan 1"
    "pic"       "vgui/extras/attract01"
    "command"   "sv_lan 1"
}

"lan"
{
    "title"     "Connect"
    "subtitle"  "192.168.1.1"
    "pic"       "vgui/extras/attract01"
    "command"   "connect 192.168.1.1"
}

"lobby"
{
    "title"     "Start Coop Lobby"
    "subtitle"  "Kind of pointless..."
    "pic"       "vgui/extras/attract01"
    "command"   "map mp_coop_lobby_2"
}

Select Start LAN on the host computer and select Connect on the computer. Then the host should select Start. Make sure the IP Address is correct for the HOST.

Alternatively you can use the console:

1. Enable the console via Main Menu, then hit Options: Keyboard and Mouse: Enable Developer’s Console OR add -console to the Portal 2 launch options.
2. Press ~ to toggle the console.
3. Run Portal 2 offline on both computers
4. On the HOST sv_lan 1 in the console
5. On the other type connect <host ip> in the console
6. On the HOST type map mp_coop_start or any other MP map in the console