I've been thinking of a heated bench outside at our cabin site, but like the idea of a U-shaped bench with the burn chamber at the base of the U. Has anyone tried running 2 smaller pipelines off the base of the manifold instead of one larger one (eg. two 4" lines instead of one 8")? What issues would I most likely face other than limited length on each side?
Using two exhaust ducts from the manifold has been done as far as I know of. But two 4" pipes together is only half the cross section area of a single 8" pipe. You'd need two 6" pipes for that, which together is a little bit larger than a single 8". However, the pipes are posing friction to the smoke path, so the total wall area should be no more than the 8", otherwise you need to compensate. Here you have the reason why this isn't a good idea either: the double 6" has 1.5 times the wall area of the single 8". In order to avoid the higher friction of two ducts, you need at least two pipes of 7". The combined cross section area of those is 1.5 times so the bench need to be shorter in order to have enough temperature left to drive the chimney stack.
To sum up: two smaller pipes give more friction. Therefor those need to be shorter. When using two larger pipes in order to avoid friction losses those need to be shorter as well because of the lower gas velocity. By making the bench shorter the possibility to add enough mass is deminished. Best option: use one larger pipe unless you are into building a bell system which is an entirely different story in itself.