Well, as the phrase goes, old habits die hard. But, the more important consideration is to go with what works for you! As long as you are proficient in securing, and deploying when needed, an alternate second stage, and you and your buddy discuss the procedure before splashing, the choice is entirely up to you. Generally, yes. Because, in the “traditional” set up, the alternate second stage is usually on a longer (~40”) hose than the primary (~32-36”), and therefore more suitable for sharing..
There is a pattern emerging that changes even this, however. There are a number of university-based scuba instructional programs in the US with which I have had contact, whose approach is to have the diver use the second stage on the longer hose as their primary, and put the second stage on the shorter hose on a bungee necklace. IOW, it provides the benefits of a ‘primary donate’ approach, while employing fairly standard / commonly encountered hose lengths. I enthusiastically recommend this approach to divers who want to configure a very functional regulator system, using ‘standard’ hose lengths rather than a longer primary hose (5’ or 7’). In fact I demonstrate the approach in OW training, using shop regulators with 'standard' hose lengths, and adding only a bungee necklace (although, adding a 70 degree adapter to the longer, now primary, hose, helps.In the scenario of an out of air diver and a potential donor with a functioning reg in their mouth, the diver who is clearly and unambiguously in the most distress is the OOA diver. If you donate your primary, and find you alternate is not working, you actually have some time (at least a few seconds) to see if there is a correctable problem, then work out a way to share the one functioning second stage with the formerly OOA diver, if there is not one. In reality, the assumption is that the alternate second stage may not be working because it has been dragged through sand, etc. So, yet another advantage of keeping it safe and secure on a bungee beneath your chin is that is it rather unlikely that it will be dragged through anything, PLUS divers that use this set up will periodically breathe from that bungeed alternate to confirm that it is working (as Centrals noted in a previous post).
But, functionality is certainly not the only argument. If I am diving in a (properly trimmed) horizontal configuration, an out of air diver approaching me at the same depth is unlikely to be able to see an alternate attached somewhere in 'the triangle'. What s/he WILL see is what is in my mouth. A very reasonable point! I cannot tell anyone else what is best for them. All I can do is tell people what I do, and why. As an Instructor, I can offer to share equipment at times with students / fellow local divers. I can offer to get in the water with them, if need be, to help them try different configurations, be it regulators, or backplates, etc. But, what I personally find to be best for me is just that – what is best for ME. Under no circumstances will I say that what I do is the only way to do things, nor will I say that it is the best way for anyone else. Now, I will also acknowledge that - with regard to use of a bungeed alternate, and a primary on a longer hose (be it 40”, or 60”, or 84”) - I also know of no reason why that configuration should not work under any situation in recreational diving, irrespective of geography or diversity, other than a matter of an alternative personal preference.
So, I use a long hose (7'), bungeed necklace configuration. The purge cover on my primary second stage - the one in my mouth - is yellow. I started using that as a recreational diver, long before I pursued technical dive training. I know how to use that configuration. I make sure that people who dive with me know how I am using it. I believe in, and actively practice, primary donation in an OOA situation. I use identical second stages on both hoses, with venturi controls and user-adjustable breathing controls. The two second stages are tuned to the same cracking pressures. (I also do not believe in using cheap, lower performing, needlessly detuned "octo's".) My wife dives exactly the same configuration. In fact, we regularly air share during Caribbean dives where we are both using an AL80, because she has a lower RMV than I do, so I breathe from her long hose for 5-10' of a dive, while we are swimming along a reef, and we end up surfacing with essentially the same gas supply. Using a 7’ hose facilitates that practice immensely. But, that is a matter of convenience - sharing air is not essential (although the opportunity to regularly practice the skill is an additional benefit). If we didn't do it, I would simply ascend before her.
What other divers may choose to do is entirely up to them. It does not affect me; it affects them. I make every effort to dive in a self-reliant manner.
