I have an existing pre-built fixture composing of 12 cables each going to a single enclosure/bottle.
Each cable has 4 cores/wires.
My understanding of Paul_B's solution fits 12 "strands" of 2 physical LED's to create 12 "sudo/pair" pixels. Din at top of each "sudo/pair" Dout at bottom of each pair so the sketch would call LED 0 but light physical LED's 1 & 2 in enclosure/bottle 1 then call LED1 to light physical LED's 2&3 in enclosure/bottle 2 and so on. 12 physically discreet LED pairs compatible with existing cables.
My understanding of yours, now, is 2 "strands" of 12 LED's uncut, stuck back to back with Din at the top of both strands Dout at the bottom. The physical limitation of the build means I need 12 physically discreet pixels. 1 pixel (or pair) going to each enclosure.
I understand visiting external links provided in help requests is frowned upon but in this case the instructables build pictures might help. just visualise 2 ws2821b LED's for every 4 pin RGB common anode LED in the build.