Time for an update! Overall, good news I think, they're both still alive in any case. Lija's suggestion of an electrolyte bath seemed to work well. The footage over the next few days showed activity - not a huge amount, but perhaps more like what I would expect. I haven't actually seen feeding, but it's possible as there are a number of places in the vivarium not covered by the camera so that's inconclusive.
The only small worry was yesterday morning after the lights came on I found the slightly smaller individual wondering around the floor looking emaciated and dehydrated. I went to grab the things I needed for another electrolyte bath but by the time I came back he/she had found the water bowl and was sitting in it so I poured the electrolyte solution in there instead. I didn't think he would last the day as he looked really unwell but to my surprise when I came home from work I found him sitting on the edge of the bowl looking really good again:



I didn't take a 'before' photo, but trust me, he looked really bad.
He hasn't moved a muscle since then and is still perched on the water bowl this morning 24 hours later. The other one also hasn't moved for a couple of nights:



Really hard to tell what's going on with these guys. When in the roosting position, they both look really well (and quite chubby), but when walking around they look very thin and under nourished. I think it's clear that they are struggling to stay hydrated so for the moment I've switched the fan off completely and maintaining the humidity at 60% with the fogger and misting to see how we go. Feeding-wise it's possible they are eating and then sleeping for extended periods afterwards which I believe is normal behaviour? I'm going to try and set further cameras up to see if I can shed some light on this.

Incidentally, as far as sexing is concerned. I was aware of the nose profile method with sauvagii, but I didn't know that applied to bicolor too. Here's a better shot of the profile of the slightly smaller one:



To my untrained eye, looks like maybe I have one of each?