Just wanted to post this in case anyone is interested in doing something similar, I added a UFX III to the UFX+ in my studio to future-proof any increase in IO as an aggregate device on Mac OS and it works like a charm. No dropouts experienced so far, recording dozens of tracks from each interface simultaneously without issue. Latency seems to be increased by .1 ms at a 256 sample buffer...so completely negligible. The UFX III is even connected through a CalDigit Element TB4 hub. All clocking connected from the UFX+ via BNC cables with t-connectors. I'm pleasantly surprised at how little of a headache it was, to say the least.

Mac Studio M1 Ultra, Ventura 13.3.1 OS