:) The problem with Sven-Coop I believe (if my memory is too be trusted) is that the levels/enemies don't scale for the number of players in the game. I know your wanting HL:S, but considering it's a pretty shitty port, I would really go with Sven Coop. Not only that, but Sven Coop has mad amounts of really fun user created Half-Life style of coop maps.
Also, if you are feeling up to it, you could purchase the Half-Life 1 Anthology for $15.00 on steam and I believe Sven Coop supports Opposing Force and Blue shift as well. If you have the original Half-Life or can spend $10.00 to buy it, the Sven Coop is almost absolutely your best way to enjoy co-op play with the original Half-Life. Most of them seem to focus on HL2 or custom coop maps.
But to be honest I am not sure that any of them were specifically made, or have the ability to play Half-Life: Source. Said: The only ones I really know of are Synergy, which you have already stated, Obsidian Conflict and I believe Garry's Mod has a way to playing coop.
