With the MM controler, you don't have any choice to use an external BEC as the CC one, becaus ethe internal MM BEC don't support 4s li-po.
So if you have wired 2 x 2s in serie, you have a 4s, right?
So remove the red wire on the 3-colors wired (red, orange and brown one who exit from the controler) and isolate it. Plug it to the chanel 2 of your Rx.
Now, take your CCBEC and plug it in the BATT chanel of your Rx.

The black and red input wird of your CCBEC must be conect to the black/red input wired of your controller, so when you plug the li-po pack on your controller, the CCBEC will be energized.

Try it and re-post here to tell me if all is OK!