2 UDS will pay for themselves in about 3 cooks compared to the amount of fuel you will go through keeping a double barrel cooker hot. They are serious fuel hogs.
It will be hard to make it reverse flow with the 2 pipes going to the top barrel on different sides. I saw one while I was researching them. It had 2 pipes but on the same side.
I built a premium quality double drum, stainless at that..it has NEVER been fired again since the UDS was built, not once, good thing its stainless..as its just sitting out back.