Separate or shared quarter wave volumes for multiple drivers?

Ok, if we imagine we are building a single volume quarter wave for a MTM o TMM, we use the center between the two drivers as Zd.

But what about the Ariels. They have separate lines and they do not use Zd.

What are the weights of significance of using separate lines?

What if we build an MTM bipole (front + rear MTMs). What would make more sense if we have to use two separates lines overall?