Show me a rule where it states you have to have 2MC's to the best of my knowledge there isn't one, the only rules regarding formations are, at least 1ST, 1 GK and 2 flank players, they can be DL/DR, MR/ML or AML/AMR.
I rarely play with two MFC's and in most game dominate possession by around 20%

As an example this formation is as legal as they come.

--------------------GK
DR-----------------DC----------------DL
---------DMC--------------DMC
MR------------------------------------ML
---------AMR---------------AML
--------------------ST

Or close you defence up to 3 DC's still legal.