So I've gotten into the habit of downloading a lot of custom maps lately, because it really adds that replay-ability to the game which it didn't have before. I can play single player maps fine, just putting the .bsp file into the correct folder.

Where do I put the demo (DEM) files?

And, also, I'v been trying out doing custom multiplayer maps, where both me and my partner have the maps, and when I'm Atlas I type change level "mapname" into the menu but it just drops both of us. What's the problem?
What do I do?

cavernousinside wrote:
Where do I put the demo (DEM) files?

...\Steam\SteamApps\common\portal 2\portal2

Hopefully someone else can help you out with your co-op issue, as I haven't played coop at all yet.

cavernousinside wrote:
And, also, I'v been trying out doing custom multiplayer maps, where both me and my partner have the maps, and when I'm Atlas I type change level "mapname" into the menu but it just drops both of us. What's the problem?
What do I do?

Open the console and type "changelevel nameofmap" with out the quotes. You have the be the host (Atlas) for this to work.