6 hours wasted... and today I fixed it in 5 minutes by putting an HDMI splitter after each cable box....

Thanks to Zonn at Zektor once again for setting me in the right direction - I had totally forgotten this cable box model was one of the ones that had that issue. Almost every job we do gets Verizon Fios so we rarely even deal with Comcast boxes anymore. Shockingly, almost every time we do, there's a problem!