Well since both Ferrari and Maserati are owned by FIAT it was more friendly corporate sharing in my opinion. Also the Enzo was never raced so it was Maserati who stepped up to prove that the chassis and engine could be a winner. There is nothing like having a race pedigree which is something the MC12 has yet the Enzo does not have. PS, I have a standing personal invitation from the designer of the MC12, Frank Stephenson who is now with McLaren, for a private tour of the McLaren complex. Some reading you and others might enjoy: https://en.wikipedia.org/wiki/Frank_Stephenson
Not that cut and dried. 50% of Maser was actually sold to Ferrari in 1997 and they made it the luxury division, and gave it advanced production facilities, hence the MC12 shareed components with the Enzo. That changed in 2005 when Maser was moved to be partnered with Alfa Rome to become Maserati and Alfa Romeo Group under Fiat Group. They are autonomous design and production groups with only general corporate oversight from Fiat S.p.A, the larger entity which entails the various groups covering areas not directly related to the automobile industry.