If the cabin you like for your 14 days cruise is only available if book as two separate 7 days cruises, you can call your CVP or TA, and just tell him/her the cabin number, and the CVP or TA can easily get approval and book the cabin that you want. Can do the same in reverse also. Meaning you want to book two 7 days cruises, but the cabin you like is only available if book as 14 days cruise.
I have done both ways many times. I did it for b2b, b2b2b, etc., so I know it is doable. I called my CVP, and he easily took care of it every time.