This has nothing to do with xlights, it had to do with my channel mis-labelling for my 2nd SS Hub. I erroneously programmed the SSCs on Hub 2 to my actual LSP channel, which were channels greater than 4096.
Interestingly enough,they did work, but at a lag at times, which is why it wasn't easy to diagnose. Re-programmed SSCs to channels 1-4096, on SS Hub 2, and we're good now.