First, why are you using separate channels? You should and could chain the strips… even if you want to use an expander board, with 8 channels, you can do at least 3 strips per channel (and really more)…
I’m guessing you wanted to avoid running a data line on the end of one strip, to the next strip… but you could run it underneath, right? If you are coating in epoxy, it’ll be invisible anyway. Yes, it would mean zig zag wiring, but the mapping fixes that anyway, so… but if you have the expander, I guess…
I know @wizard used multiple expanders for his Volumetric Cube, so I’m guessing you can wire them up, but I think you have to change the address of each board, different serial bus address (so it’s shared data lines from the PB, each only listens for it’s own address)