I think it is harder. I'm not positive of how mmr works, but I think it essentially gives you worse teammates to balance out the game or something. I'm pretty confused as to how its handled.
I would expect that a duo is matched with another duo of similar mmr, but it may just match a duo to a duo, and then the teams up to match the mmr as closely as possible. Which can easily create rather large imbalances.
I think the only good way to duo is if both of you are significantly better than where you are in the ladder. Its just my opinon.
I don't think the matchmaking system is very effective at the moment.
I have a post on why, and some ideas as to how I think it could be resolved. They're pretty extreme ideas, but I'm pretty sure it would work. Any input would be appreciated:
Fixing the Ranked Ladder System