Building a wall using 2 separate boxes

[Slapz]

When building a wall for say 2 18's is there going to be any real difference if i use 2 separate boxes one for each sub compared to using one big box if both subs end up with the exact same cubic feet and tuning? i'm not talking about how one may be easier to install or how 2 boxes will take up slightly more space etc, i am speaking purely from a volume level is there going to be any diffrence if they share a box compared to having their own identical boxes?