RimWorld Together is the mod that does it. What it is, and what we have not tested.
Multiplayer
This mod contains no multiplayer of its own. No client, no server, no networking of any kind.
The mod that does multiplayer in RimWorld is RimWorld Together. If you want to play this with somebody, that is the thing you install.
What RimWorld Together does
Each player runs their own colony, on their own map. It is not one base with several people in it.
What it links instead is the stories: by its own documentation, offline visits and raids, and item and pawn exchange between those separate colonies.
That model is why this mod fits it at all. A branch office is already a thing you run on your own — your facility, your gate, your coordinates, your research, your ledger. Two players are two companies, not one company with two managers.
It has its own requirements. RimWorld Together needs Harmony. That is its requirement and not ours — this mod uses no Harmony and needs none.
What it is not
| Not a shared colony | neither this mod nor RimWorld Together provides one |
| Not a shared map | there is no shared map in either |
| Not synchronised research | research is not synchronised; each branch researches on its own |
Those are not limits we added. Separate colonies are what RimWorld Together is, and it says so itself.
What has actually been tested
Nothing. No session has been run, by anybody, with these two together.
That is a statement about what has been verified, not a prediction. Specifically untested: separate starts, an offline visit, a supply or aid exchange, reconnecting after a drop, and whether anything company-specific travels between players at all.
Assume nothing company-specific transfers. Keep each company's facility, gate, discoveries, research and ledger on its own branch, which is where they live anyway.
If you try it
Match the exact game version too. Rimrooms is being prepared for 1.7 unstable / Waylith while retaining 1.6. That does not establish RimWorld Together's 1.7 support. Check its current release requirements and qualify the chosen client/server pair separately; keep both branches' saves backed up.
Everyone needs the same mod list. RimWorld Together's server does not enforce mod order or settings, so matching them is on you and the people you play with.
Back up your save first. Treat single-player as the supported experience until somebody has run a real session and written down what happened.
Report what you find — see Links. A real result is worth more here than anything on this page.