Yes its a forgotten mod ev eryone forgot about its called portal multiplayer beta v2. Since no one finished the multiplayer stauts. i will work on it. if you would like the mod, Link on bottom:

{链接已删除}http://www.strategyinformer.com/pc/portal/mod/23410.html

And to install:To install: Go to the directory C:\Program Files\Steam\steamapps\\portal\portal and copy and paste the contents to C:\Program Files\Steam\steamapps\\portal\portal Backup. Extract the contents of your portal multiplayer.rar to somewhere you can remember like desktop. Open the files and copy/paste them into C:\Program Files\Steam\steamapps\\portal\portal. To go back to 1p you need to replace the files with the backed up ones.