My friend and I grabbed a few custom co-op maps off fileplanet. We tried to play two of them and they worked just fine up until we had to load the next chambers. The game just gets stuck on the loading screen for both of us. We're both able to hear ourselves firing portals while we stare at the loading screen too. Does anyone have any idea whats happening?

The maps's exit starts the loading screen, but because they are not connected no next map gets loaded. You will have to go to the lobby before going into the exit tubes.

The lobby? Like the hub that you load the custom maps from? I'm not quite sure what you mean.

Wait I think I get it. Are we reaching the end of the maps? I didn't consider that. One of the ones we downloaded was apparently only one test then. I thought it was trying to load the rest of the chambers for that particular map.

I don't know if the maps you downloaded are connected to each other. If not, you will land into the lobby (if they made it so) or there just happens nothing (what seems to be your problem now).