Dear Moses,
If you look into the SAP Help, you would come through the following:
"If one of the account assignment elements fund, functional area or grant was activated in Customizing of Public Sector Management, all company codes of a controlling area must be assigned to one FM area"
There is 1:1 relationship between FM Area and Controlling Area. Which means you can have as many as FM Areas in a Client subject to the condition, that you have same number of Controlling areas. In Nutshell, you can assign as many as Co. Codes to FM Area as far as they are assigned to same Controlling area.
Since FM requires Cost objects (Such as Cost Center, Internal Order, WBS etc) as a base to identify the Costs in FM, you cannot assign one controlling area to more than one FM area.
Coming to your second query, the prerequisites of creating more than one FM area, and ultimately more than one Controlling area, in a client can be operations of Company in multiple countries, which can lead to dealing in multiple currencies and so on.
If, for example, Co Code 1000 is operating in USA with Currency USD and its subsidiary Co Code 2000 is operating in UK with Curreny GBP, then it is suggested to create separate Controlling and FM Areas for proper budgeting and other related processes.
I hope the above clarification serves the purpose.
Regards,
Amar Lal