How possible would it be to just create zones within which items and entities would be read, transmitted, despawned, and then spawned in your friend's receiving zone? Not exactly multiplayer, but both people could use the same world if they wanted, to make it somewhat parallel, and trade things and needed workers. Separate means could be used for viewing one another's screens if desired.
What do you mean by "same world"? If it's asynchronized, then it's more like "parallel playthroughs". If it's synchronized, then that's a very hard problem and might take as long as DF itself.
Assuming both players are using vanilla raws and the same version, this sounds possible but will probably have negative side effects. Any links to history (tracked kills, being a histfig, being an artifact, etc.) will be severed, and it will probably be difficult to restore them if they return. If it's asynchronized-parallel, it might seem possible, but histfigs and artifacts are created over time so you will still need to sever links to histfigs that don't exist in the other world. And what if somebody made an artifact in Aleph but not in Bet? Should it still be linked to them? This shouldn't be an issue if it's synchronized, but at that point you might as well just implement sending trade caravans. That's minimally difficult compared to implementing synchronized multiplayer.