One issue that I've struggled with on TMCC and DCS upgrades, and even on some stock Lionel dual stack steam is getting the smoke to flow reasonably even between the two stacks. It seems like they're never in sync, either the front stack or the rear stack has most of the smoke. Some of the worse offenders are the otherwise really nice JLC Challengers! Another one that has very poor distribution is the LionMaster T1 Duplex and the other LionMaster models with the same stack design.
I have a 3rd Rail Big Boy on the bench for an upgrade, and I decided I needed to figure out a different scheme. I happened across the Hot Box Reefer smoke unit body on the Lionel parts pages and an idea hit. It has a hose coming out the side, and there's a PCB with the smoke resistor with no stack hole. However, I didn't want that, so I used the standard rectangular smoke unit PCB with the hole. I mounted a short piece of brass tubing in the stack hole of the PCB, and then I cut the side hose to size, put a spring inside it to keep the hose from kinking and shutting off the flow, finally I inserted a short piece of brass tubing in the end of that one.
The final piece of the puzzle is that neat 3D printed stack assembly that Bruk nicely had printed for me, it was a perfect fit in the stacks of the Big Boy. I use it to align the two brass tubes, and the whole assembly will be inserted into the boiler of the Big Boy.
The mod works perfect, both stacks have a generous smoke flow, it's the best solution I've come up with so far to even out the smoke!