I've noticed that some have been confused by what I mean. You are supposed to send a copy of your map to the other person a copy of the map. A server is not needed. You are supposed to build whatever you want within the time limit. Also, it helps to actually read this stuff.
Recently, I have wanted to build a custom map, but I got bored and started thinking about doing a co-op. Well, I don't have a server or skype, that's a bit bad. But, anyways, to my idea. First, pick a partner in which you can switch between maps. After that, one of you starts making a map, which could be anything (custom map, sculpture, etc.) After a few days to a week, you give a copy to the other person who should work on it. Whatever you do, don't tell what you're making. When the partner thinks he/she is done, the map is done (sort of.) Now you can only make small details (storyline, quests, etc.) The partner can make something totally different than yours. If you upload the map, make sure to say that you did this so that people wont say that you copied the other person. Hope you liked my idea. If anyone would like to do this with me, tell me so that I can either start a map or wait for you to.