Maybe you know things that I don't, but I have developed briefly on UFO2000 before developing on openxcom, so I know both systems technically rather well, and I would certainly not say that mixing both games is easier than implementing multiplayer in openxcom
Adding multiplayer is not developing things from scratch, it's adding network code on top of the current battlescape code. Code that sends the action of the opponent over network, instead of the action being driven by the AI and send the changes in the world to the client, so the games on both sides are in sync. That's the basics :p
edit: main problem probably with making ufo2000 and openxcom work together is, you need to write some kind of interface between the two, which might be possible, but doesn't sound like an elegant solution. Also it needs code changes in UFO2000 which is on totally different repository, has a totally different structure (it's very very ugly code compared to openxcom :p). And in the long term, everytime somethings is changed in openxcom, you might break the interface with ufo2000, so a new version of ufo2000 needs to be released to fix it again, etc....
I made the UFO2000 installer, do I count as a developer?
You might not need to create a direct interface between both programs. IIRC UFO2000 has some kind of save/script support, so a simpler approach would be to just generate a file with the mission data that would be loaded into UFO2000, play a game, and put the results back in OpenXcom.
Anyways that's besides the point. Supporting multiplayer is not an "implementation" issue, it's a "gameplay" issue. This is the kind of discussion that goes around and around over and over again, people always think "hey what if the other side had a human?", that it'd be that simple, that it'd be fun. But I don't think playing X-Com multiplayer is an enjoyable experience. You can't just "tack on" multiplayer to it.
Let me clarify. X-Com is designed to be a struggle, a challenge for the player. The odds aren't in your favor. You have to fight your way from the bottom until you can finally top them. It's a reverse difficulty curve where you start as difficult as possible and only start easing up as you gain leverage on them. It's a brutal yet enjoyable experience, but it's not balanced. The aliens are better than you, stronger than you, more powerful than you. By default it's an unforgiving punishing game and that's just with an alien AI. With an alien
human, well you might as well just sit down and take it because you've pushed so far against you you're gonna have a hard time ever winning or having fun. Blaster bombs and mind control are insta-game-overs. I'm not making this up,
people have tried this.
Basically if you wanted proper X-Com multiplayer you'd have to redesign the game from scratch to make sure it's balanced and fun for both sides, like UFO2000 and UFO:TTS did, and that's not really within our scope right now.