It is impressive to see how the number of posts have mushroomed over the past 24 hours. And, I can certainly appreciate where many of the posters are coming from with regard to equipment configurations, optimal hose lengths, buddy practices, diver attitude, etc. But, I won't go there, as others are covering those issues quite thoroughly.
All double diver I have seen use bp/w setup. Is this a must? Is there other options out there?
I will add my thoughts, which probably echo several other comments.
1. You have received a number of great suggestions for non-BP based doubles configurations. So, the answer to your original question, 'Is this a must?', is 'No.
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.
3. With regard to backmount doubles, several posters have mentioned the Transpac. I was really skeptical of the stability of backmounted doubles attached to a Transpac until I had to use a Transpac with my BM 130s a year ago. I found it
very stable in the water, and would therefore suggest it as a reasonable option. With a little rigging you can add a buttplate to a Transpac and have a SM rig. I have a Zeagle Ranger and have put doubles on it as well. Of the two I slightly prefer the Transpac with BM doubles, but both work. I would recommned that stabilizing plates be used with either unit. These are not particularly expensive, though.
4. I can appreciate part of what I perceive to be your dive buddy's mindset - go with the minimal equipment necessary for a dive, relax and enjoy the time in the water, don't get hung up with un-needed equipment or stressed out by procedures.
According to him, the DM their group hired as a tour guide had very good in water skill ... best buoyancy & control with only rental reg and jacket BC. The conclusion for him is long hose, bp/w ... (so called the tech setup) are not necessary.
And, he is absolutely right - they are not 'necessary'.
Ever since that, he is trying to do what that DM do, including doing safety stop upright with arms across in front of chest, and a few other "techniques" he picked up.
I have seen any number of Caribbean DMs who dive the way you describe the DM in the Phillipines diving. It works. I do it myself from time to time - go vertical, cross my arms (and my fins), and just hang in the water, or drift along. I enjoy it, and the freedom it offers - I can almost doze off as it is so relaxing. And, I can do it in my BP, or in my jacket BCD, or my Ranger. (And, frankly, I enjoy solo diving because of the freedom it offers as well. I don't have to worry about whether my buddy knows my gear configuration, or I know his. I don't need to worry about a long hose for my buddy, and I know exactly what I have to do in an OOA.)
5. The challenge for you is finding a way to meet your interest in moving into doubles, with your buddy's interest in a minimalist equipment approach, and do it safely. That is not impossible by any means, but I suspect it will require a bit of effort, and evolution.
