The only way that I could see 2 cook chambers being fed by 1 fire box is if you were only using 1 cook chamber at a time. Heat/air will always travel the path of least resistance so it wouldn't work with 2 cook chambers and a single fire box. I believe that it was towtruck that built a center feed cooker that was 1 fire box with basically 2 different chambers and the heat always traveled through the easiest route. I can't remember how he fixed it but I think he may have came up with a solution.
What if I wanted to run both cook chambers at the same time and treat it like one big cooker. I figure the firebox would have to be upsized but how would I do the opening between the two chambers?
As Big T mentioned, two separate cooking chambers usually doesn't work out. You could definitely add length to your current cooking chamber however. You would need to calculate the new cc volume and increase your throat, baffle plate gap, exhaust stack, intake vents and firebox to match. At that point, you have pretty much built a new pit so it just might be easier to start from scratch and actually build a new pit.
You could use the new tank as a warmer box by mounting vertical above the firebox if you just needed more space. A warmer box uses "secondary heat" and is not counted in cook chamber volume in most cases. Problem is a round warmer box in a pain to make and just a lot of wasted space once you put square shelves in it.
Yeah. Doesn’t sound like it’s worth the trouble. Prob just build a second smoker out of the other tank. Then at some point sell one or both and build a 300 gallon. It’s not like I need it. I just want to run with the big dogs for fun.