Random theory: The single-tank diver may have been used to following other divers & just surfacing whenever they surface. That's of course a recipe for disaster in general, but even more-so if the dive-buddy has double your air.
Plenty of blame to go around here and lots of opportunities to learn for both the sidemount diver and diver #1. The gas management of diver #1 was the primary cause of this and perhaps diver 1 was distracted by their own filming. I would not be surprised if other videographers/photographers had stories of suddenly realizing that they had not checked their gauges in an inordinate amount of time, as well. From my limited experience of photographing recreationally, it can be easy to become distracted/hyper focused on a subject.
I find it difficult to excuse the sidemount diver for turning away from diver 1 when diver 1 was clearly trying to signal something. Perhaps diver #2 was distracted by the task of the dive and became hyper focused on completing the task rather than communicating with his buddy. Granted, as MaxB stated above, Diver 1 should not have OOA in the first place.
If I'm playing a video-game, I often have little concept of time. Underwater photography could be somewhat similar. I just got an action-camera, but I have it setup as record-and-forget and am not really doing deliberate underwater photography.
There are EPGs and air-integrated systems, which can alarm, when air gets below a certain PSI. Pony bottles are also a good option for redundancy. Neither of these should substitute consistently monitoring your air. Before I started diving, I was always worried I'd have problems being surprised by low air. So far, I haven't come close, and have gotten fairly used to intuitively knowing about how much air I have left.
I think that if I needed air and my signals were not immediately understood and acted upon by the other diver, I would have just grabbed the reg from his mouth. That would be a pretty clear sign of my desires.
I actually tell new buddies to do this to me. I have an Air 2, so I specifically tell them, "hey if you need air, just take the reg from my mouth, I PROMISE I will notice right away". I follow that up with: "The pony reg and the AIr 2 are for me".
Agreed, signal first (if practical), but then grab the reg if the response is slow. I'd also add make sure they see you grabbing their reg, so they're less likely to breathe in water or bite off the mouthpiece.
I've heard these types of devices (2nd stage attached to inflator) can be much harder to manage in general, which can be complicated by a panicking out of air buddy. As a general statement, people who use these devices should practice using them, including managing buoyancy while breathing from it. I have a similar but cheaper device I acquired with some regs & since it's small, I just stashed it as a spare 2nd stage in my save-a-dive kit.
Something else I do, is cover the donate-hose with something like this:
25 FT 1/2" Green Expandable Wire Cable Sleeving Sheathing Braided Loom Tubing US | eBay It's some slight extra protection, makes it more obvious which hose & reg to go for & easier to see in a potential long-hose entanglement scenario.
