I’m somewhat familiar with Unity3d, and done a few tutorials in the past and found them pretty straight forward, but it’s been a while and I believe Unity has updated quite a few times since then. Just thought I’d come here and see if anyone can throw out some suggestions that will help me on my way. Thank you!
There was a multi-player FPS game in the learning section, I think it was Christmas themed but it would be a good starting point. Assuming you mean co-op via online.
If you mean co-op via one device then just find the learning section that covers shooting and then duplicate the player and look into how to split the display (camera setting width/height). Then duplicate the code or make public the controls and set up the 2nd player controls.
Basically check out the learn section and use the API manual to work out the changes needed, and ask back here for any tips or advice.
Could be a good starting point - Let's Try: Shooting with Raycasts - Unity Learn
There are lots of online FPS tutorials and even a starter kit on the Asset Store.
Look up Gamer to Game Develop S3 on youtube. Also a thread about it on this forum somewhere. Fantastic, very comprehensive FPS tutorial. Don’t know if it covers MP – but pretty much walks you through every aspect of the game, code-wise that is. You’ll need to get your own assets.