That would very quickly not work.
Because well, players play differently. Or maybe one player has a slower computer. He would lag behind, and be effectively out of sync. After all, he can't send caravans (they would arrive in other people's pasts) and other players sending caravans to him wouldn't work either, as he'd have to wait a decade before they get here. Additionally, there's not much point in doing it, there's no real gameplay value in this.
It wouldn't be mandatory, just an option. There could be some sort of time cap for how much forts can get out of sync, but faster computer vs slower affects most real-time games, so you might just leave it to the players to handle the differences, if they want to play together in a meaningful way. Well if you could make offers/demands, and send raiding parties or just helpful immigrants there could be gameplay value.
Even without the time cap, the waiting a decade isn't a problem, if the other player could drown you in golden goblets, or send an angry steel dwarf squad to you, while you are still collecting berries would be, though the fort in the past sending things to the other fort in the present would raise issues, maybe dwarven time travel?
Of course this could quickly get more complicated than its worth, so Adrian's idea would be a more reachable goal.
Also people claim that Toady has said no multiplayer, I couldn't really find the sources, other than some ancient posts where, he states that the code isn't in shape for multiplayer, or that he isn't well versed in networking, but nowhere a definite we will never do this.