In StarCraft: Survival Mode you can play with up to two players in the co-op mode. To play Co-Op, you will need to do a few things.
Go to your StarCraft installation and inside the maps folder, look for downloads. In this folder you will need to copy all of the Survival Mode maps to. Make sure you copy from the Co-Op folder and not the 1Easy, 2Hard, or 3Zerg Survival folders.
If you are going to play over Battle.net, I don't think the download time will be that long.