There is a page up with info on playing the normal Half-Life single-player maps co-operatively on SvenCoop - Atomic Half-Life. I don't know if there's anything on the page that's changes since the SvenCoop stuff was first announced a while back, but questions on "how do I play co-op Half-Life" show up in the mail here all the time, so there it is (described as "far from a perfect method, but it's fairly playable").