While I agree with remarks about the difficulty of implementing world scale multiplayer, I don't understand people saying anyone would be foolish to even attempt this. It's one thing to deem a task difficult, it's another to actively discourage people from attempting it. Would you react the same way if, say, two brothers set out to make a world simulator with ascii graphics and a schedule stretching over 20 years, in 2019?
Also, there are different kinds of multiplayer. Two people managing different forts on the same world would be difficult, but the following are feasible or already done (check Warmist's thread on multiplayer)
-"Hotseat", i e. multiple players take control of the same game instance in a cooperative setting. Already done, check dfterm2, dfterm3, webfortress (although they haven't been updated in a while)
-Two people managing two different fortresses on the same embark site, possibly being hostile to one another. It requires the game to render two different views of the screen at the same time and some work has been done on that.
-Different people managing the same fortress, but with different screens for each person (so, let's say, one takes care of labors, another one may manage the military, and so on). Same as above
-A player runs an instance in fortress mode, and other players control a single unit arena style. Pretty cool in my opinion, and already doable.
-Mutiple players controlling multiple adventurers. Requires a fair amount of bodyswapping for each action and each player, but doable. When the new release comes out with a proper implementation of multiple adventurers it'll be presumably easier.
-Otherwise, I guess a turn-based fortress mode would work. Two players take turns retiring, playing and unretiring their fortress, build and send squads at each other. It'd need to catch the "wait two weeks" process and prevent it, and I'm not sure whether offsite squads are properly tracked and what happens if a player-sent army arrives at a newly unretired player-controlled fortress, but if everything is accounted for by the game it could be neat.
I'm sure I'm missing lots of possibilities there. Don't dismiss the idea out of hand.