The thing about MP is you really can't plan what your going to do, you have to improvise based on your opponents moves in order to win. You don't go into an aikido server and just plan to manipulate your opponent in a certain way and think it will work because it wont always go according to plan. I'm not sure what you mean by communicate with you partner either. Personally I think MP is harder because you are never really too sure what your opponent is going to do next, in any mod. In SP it's easier to take your time and think out what you want to do and make a great replay, after all, Uke isn't going to move unless you move it.
Both of them just require time and patience, I believe.
But I believe SP takes more time because you have more tools, such as no set turnframes and the ability to move Uke. These make for a more complex replay that will take more time to do.
MP is easier because as long as you have a plan of what to do and communicate with your partner, things should come out at least close of being right.
I think MP is harder cause you do not have the shift space for 1 frame and you can't go back and edit it with out turning the second player into uke.
In MP replays, you get away with stuff that is considered shitty in SP replays.
But you have to consider that you're fighting with another player, so the quality of the replay comes from the skill of both players, luck, the flow of battle, and (if it's a co-ordinated battle, as in both players are working together to create a replay) co-ordination between the two players.