This is the information I was seeking. The yellow Mares Octo reg I see almost everyone with is used due to cost savings rather than use a more $ reg. Still I wonder why the tech divers use a full size reg instead of the smaller Mares type as a second? Is there any reason one should consider the more expensive option?
Well a few reasons why.
First is that as a recreational diver if you go for your Octo and it doesn't work, another option is to go for the surface and as long as you breath out and don't exceed ascent rates all should be well (relatively speaking). With tech diving you usually don't have that option as you are probably racking up deco time and you are at significant depth preventing travelling to the surface on a lung full of air. Thus you are more dependant on your gear and so tend to buy better quality on the basis it will (maybe) be more reliable.
Another is that depending on cylinder configuration you may need to breath equally from both regs during the dive (when using side mount or twin independents) Given you are using the regs equally its reasonable to assume you don't want to be spending half your time sucking on a dog of a reg just to save some money. More comfortable and better to have 2 quality regs.
The reg on the long hose is the one you normally use if on twin manifolded tanks. Reason being if someone is out of air, you are handing them the reliable reg immediately, the reg around your neck is the backup, which one hopes will work for you, and as its at your mouth, easy to get to in an emergency so as long as its reliable all is well.
Given the additional depth for tech diving, better quality regs make diving easier and less stressful. A reg that breaths easy at the surface maybe a bit of a dog at 60m.
Whether the hose/reg is yellow or black is of no other significance other than to show someone in a panic a reg they can grab as its your "spare", and to denote the one you should normally use. In saying that you could use 2 standard regs with black hoses etc. It has become an accepted dive standard to colour the hose/reg by virtue of use I suspect. It hasn't been applied to DIR that I am aware of. I like using 2 quality regs but on my recreational gear I have the standard setup which I use to fit with everyone else, which makes life a bit easier in an emergency.
I also have my deco reg hose green to show Nitrox and white tape on the top of the reg (as well as the cylinder Max Operating Depth and Nx %) with the MOD of the gas on the white tape. This also prevents confusion.