Hardware designers put in what's needed, they wouldn't put in dual pipes for the looks. They probably want the heat dissipation to occur evenly on both sides of the phone, rather than a single larger pipe making one side of the phone hot.
In Any case, I'm not having Any problems with my Phone with regards to the excessive heat. Sure it gets warm once in a while but not like my Z1 did. So it steeds to be working.
I bought this Z5 Compact two days ago. No excessive heat at all. I played games for a while, watch YouTube for a while, took quite a few pictures. It gets warm but never hot for me.