Indeed, the game has rendering issues in Multiplayer due to the fact that it was originally built for the xbox and never intended to render on multiple viewports for PC, It's possible by just disabling the extra view port the issue would be fixed.
The issue with the Tal'Set being all deformed I don't believe relates to rendering, I believe that has more to do with the fact there's no input assigned or something isn't happening.
The floating warclub is actually the 1st player (The one I'm playing as, you can see it in the youtube video I've showed), there's more invisible stuff like the dinos on the treehouse level tons of stuff we had issues with early on the first time I worked on this and I believe some of it may have been fixed by actually disabling the spit-screen while leaving the 2nd player spawned.
Essentially just disabling the rendering of the second screen and making the game believe it only has to render the first player's screen.
Currently I'm going to debug the pistol zoom crash and see if I can't fix it,
I'll provide the patched files here (if that's allowed) once I wake up today (haven't slept been working on this all night), and then everyone can screw around with the multiplayer functionality.
Also the ATR viewer does actually load some of the multiplayer maps as well I was experimenting with that a bit.
UPDATE:Some real quick nops in relation to the area it's crashing and the pistol zoom now works

, really dirty method I should probably figure out what it was trying to access and why it failed... but regardless it doesn't seem to have caused any additional problems performing the nops that I did.
I could now reverse engineer the viewport stuff.
