The only simultaneous more-than-one work is in game mode: https://developer.valvesoftware.com/wiki/SFM/Recording_multiplayer_gameplay
Anything outside of game mode won't work with multiple people. You could just share the session file back and forth between people, but that will still only allow one person to work on it at a time. It may be possible to write Python scripts to synchronise changes across multiple people, but no one has done that yet.

It's a bit of a logistical nightmare, but can be done...

1. Make sure you both have the same assets and in the same place.
In other words, if you ported a model in and put it in a custom folder, then he would have to have the same model in the same custom folder.

2. Send copies of the session .dmx and update as changes are made.
You should always work off of a copy of the master session, that way if something goes wrong, you can go back to an earlier state without losing everything.
Once a change has been approved and is working properly, that now becomes the Master Session. You copy it and send it to your friend, and they do the same thing. Make a copy, edit the copy and then when satisfied that it's good, save it as the Master Session, rinse and repeat as necessary...

Originally posted by JJones05:

can you send your friend a copy without sending it back and can they keep your progress on thair copy?

I did make this post 7 years ago so I'm no longer invested aha. thank you though

If someone made this mod, i would be very greatful. It is difficult and time consuming to make projects by myself, but, if i had more workforce, the project would fly through production straight onto youtube. Please make this mod.