Macro is the greek prefix for "large", Micro is the greek prefix for "small". You are referring to micromanagement in which you control and manage the smallest details of a champion, or in some cases champions.
Now that my inner grammar nazi is satisfied (I'm not as mad as I sound I swear)...
It depends on what your talking about, if you mean have one champion visually comprised of two or more champions, Ziggs was originally half of a twin bomber/gunner motorcycle duo.
Controlling two actual champions as separate units is VERY hard, Lost Viking from Hots for example. It work require a lot of work, both on the player's and Riot's sides. Riot would have to find a way for such a champion to exist and be fun to play (I don't even want to imagine the coding it would require to do that).
Then the players would have to put the time and effort into learning how to play and build the champion, as apposed to blaming Riot for making a "bad champion".
I can't see if the positives outweigh the negatives, how would the UI handle two health bars? It would still be an interesting idea though, despite loving micromanaging (proved by some of the champion concepts i've posted on the boards), I would probably be terrible with them :P