2. Diving doubles does not necessarily mean backmount. I can't tell from your original post if that (backmount) is the direction in which you are going. If it is, you might be thinking that it would be preferable to have your buddy in a similar rig (or, I am reading a bit too much into the initial post). But, a sidemout configuration may offer a good alternative. It is NOT necessarily technical diving, although it is possible that having enough gas to put yourself into a deco situation ultimately qualifies it as 'technical'. The beauty of SM is that you can pursue it with a variety of equipment approaches, ranging from using a SM-specific commercial rig such as a Nomad, to simply clipping two bottles with deco/stage rigging to a BCD that has some type of chest and waist D-rings, and going diving. I could clip a couple of 40s or 80s on either side of my Seaquest Pro jacket BCD and dive a sidemount configuration it might not be streamlined, or optimal, but it would work. Dive-aholic, in various posts, has drawn a distinction between sidemount diving and diving a sidemount configuration. The latter is what your buddy would be doing - simple OW diving with two bottles mounted on the sides. I don't mean to minimize the importance of getting training in how to dive such a rig, the potential risks of having enough gas to 'get into trouble', or the conseqences of moving tanks from the back to the sides - take a back-inflate rig into the water without a tank on the back and the taco potential is intersting. Rather, my point is there are a variety of options, with broad ranges of simplexity/complexity, and expense.
One of the nice things about sidemount is that you can configure it in so many ways that your buddy may find a configuration that suits him.