I am a mason contractor whom specializes in Rumford Fireplaces. I am helping design a fireplace that had a problem fitting in a single 18" x 18" flue pipe. So I installed two 13" x 13" pipes adjacent to each other.
They start at the smoke chamber tight together with a smooth as possible connecting partition. I'm cutting out the connecting flue walls some 12" down from the chimney top so as to form a plenum in order to avoid pressure variations from wind on one side more than the other.
The two flue pipes will be equal in height and length and have the same amount of masonry surrounding them to keep temperatures even. They will be centered in the smoke chamber and their total area is 256 $in^2$, which is nearly equal to 1/9th of the 48" x 48" fireplace opening as required by all codes.
My masonry teacher built a design like this long ago as have I, and I have had no problems. However, I am being told on certain blogs that this design will cause turbulence and double the drag resistance. I'm also told that I should have used a single 13" x 20" flue with a inside area of 192 $in^2$.
I believe that having my pipes connected at the top and bottom prevents turbulence from pressure and flow differences. And that having more cross sectional area from the combined flues should easily compensate for the added drag of the extra 12" of smooth tile partition.
I also believe that in a smoke chamber, a intermittent vortex exists below the damper blade and never rises into the flue area because the flow changes to laminar flow before entering the pipes and the parting divider.
Normally, I don't use double flues. However, I really can't see how two pipes connected at both ends and being larger in combined area will hurt flow via turbulence to a point of where you would see enough flow loss to generate eddy's in the flow.
If anyone can help verify if two flue pipes is sufficient or not in this design, I'd appreciate it.
To summarize my thoughts: I don't believe the air flow will be negatively affected from using two flue pipes instead of a single flue pipe because of the connection between the two pipes and having a larger overall combined area within the pipes.