This morning I went out and did a pre-video test of the faulting. I wanted to send a new video of the fault occurring to Signature Solar.
Inverter 1 was fully indepedent from inverter two, just like yesterday, turned it on, immediate fault 52. So now that I knew the issue was still happening, I took a video of me repeating the process exactly. This time NO FAULT. You can hear my shock and confusion in the video. In my video, I started with just solar (Because this will not cause it to fault) and then scrolled through each setting, hoping that Signature Solar might see a setting that was wrong. I don't know if scrolling through the settings managed to reset something that was screwed up in there or what, but after scrolling through each setting and then hitting the inverter button, it all appeared to be working fine. No faults.
So since the previously faulty inverter was now working fine, I reconfigured it for 2P1 and the other for 2P2. Restarted them and....both are inverting. No issues. You DO NOT require PV input to each inverter in order for them to function. I took a video (Also uploaded) of this for Signature Solar to see, so that they can update their internal and public-facing docs. I was explicitly told that I must have PV input from separate sources to each inverter, but that clearly is not the case.
So whatever the issue was, it seems to have completely resolved itself somehow, magically.