Thank you all for your support and opinions. have been helpful
I already have the solution and I will be paralleled another pipe just above. Thereby doubling the flow and is a very simple task.
Welding together two drums was very complicated, because of the way the smaller drum. Make cuts on curved cylindrical surface is very complex and preferably avoid doing. Also at the weld seam on sheet metal, should be done with autogenous welder, and not have one.
With the new modified there will be an area of 5x5 "and I think will be enough.
I researched a lot about smokers do before. I have seen many fireboxes connected by pipes to the smoker.
The idea of this design is ease of use with multiple functions. smoker, grill and wood-burning oven.