Got into the closed alpha by sheer luck and have been on/off testing this with another discord user. It's nice, connecting to another player is flawless. No port forwarding/stuffty router fiddling. Everything is streamlined for player convenience. I've been downloading and testing a slew of mods and cataloging them on which mods work vs which mods don't work.
Since Vanilla skyrim is broken in some aspects, which would make multiplayer vanilla skyrim unplayable, here is a list of workable mods that don't change much, but improve on the base game to the point where it's actually playable and enjoyable with others.
Starting with Clothing mods. The clothing in skyrim is limited. Bandits always wear the same iron/fur/steel preset armor. It's annoying, so we're changing that with this little mod.
*Immersive armors - adds many new vanilla styled armors to skyrim, completely balanced, completely safe to use with Skyrim Together, doesn't conflict with anything on the list and integrates nicely with one of the perk mods we'll be using.
*Bandoliers and Bags - Adds new Craftable Bandoliers and Bags that increase your carrying capacity by 15-70 points. Can wear multiple bags on your armor giving it some pop. Doesn't conflict with anything.
Perk Mods.
*Ordinator - Honestly if you're not using this, you should be. The Perks in Vanilla skyrim don't do anything to change your character drastically, like a perk should. There are multiple trees in one perk tree that allows you to tailor your character to your liking. Want to be a battlemage? Can't because Skyrim's limiting perk system? That changes now. Want to be an archer and be rewarded for fast accurate arrows or long distant snipes? Ordinator perk system. Compatible with Skyrim Together and doesn't break anything.
Combat/Magic Mod.
*Mortal Enemies - Stops your enemies from tracking your every movement with their weapon. When they swing they won't correct their aim when you step out of the way. The same goes for you, so you have to commit to an attack and so do they. As far as I can tell this works. I've tested it and noticed a few times the enemy kinda freaked out when two people were near it, but i'm going to attribute that to Sky-Together's state of development.
*Ultimate Combat - Polishes the Combat of Skyrim. You're rewarded for timed blocks and timed attacks rather than just spamming power attacks and shield bashes. Works for you and the enemy. Lower level enemies don't utilize the mod very well but higher level enemies do. Also reworks some of the enemies to be more of a challenge. Dragon Priest's and Dragon's come with a few new attacks and are overall smarter/stronger. Dwarven Centurions come with new attacks. Groups of enemies work as a cohesive strike force utilizing their strengths and compensating for their weaknesses respectively.
*Apocalypse Magic - Must have magic mod. Makes Mages playable. Overhauls some spells and adds huge trees of completely new ones.
*Immersive Weapons - Same mod author of Immersive Armors. Adds many new and expands upon existing weapons in skyrim. All perfectly balanced.
*Tk Dodge !! - EXTREMELY EXPERIMENTAL !! So this may or may not work. I've tried to use TK dodge, but it requires FINIS, an animation extension mod. It's really finicky and it's prone to making you and your partner de-sync like crazy.
UI/QoL mods.
*SkyUI - Not only required for Skyrim Together, overall UI improvements.
*Unofficial Skyrim Patch - Fixes a lot of the bugs that Bethesda left in over their 22 reiterations of skyrim.
*Floral Overhual - Makes the game look prettier by adding in flora over clear areas.
That's pretty much it, that's as far as i've gone mod wise. I'll be testing more later down the road.