As shown in the Apple documentation, communication with a certfied accessory (certified by apple) can be made through te EAAccessory Framework. But I am getting a little bit confused because some posts tell me that it can also be made through CoreBluetooth.
I also read that CoreBlueTooth is only supported by iOS 5 and iPhone 4S may communicate with an external accessory.
The question is "Should I use both CoreBluetooth and EAAccessory Framework (I think in this case I will have to test on a combination of the device model and iOS version) or will the EAAccessory Framework be enough?".
The CoreBluetooth Framework is only used for communication with Bluetooth Low Energy enabled devices. The EAAccessory Framework is used for standard Bluetooth connections. One framework, as of this writing, requires you to be a certified MFI developer with Apple and requires special hardware (EAAccessory). The other does not, but requires Bluetooth Low Energy enabled devices (iPhone 4S) and iOS 5.0+
As to wether you should support both, that depends entirely on the use of the accessory. Each spec has its benefits and draw-backs, it is up to you to determine which best fits.
Hope this helps!