in my opinion, it doesn't really matter which side it comes off of. However, there are good reasons for longhose on the right and bungeed on the left. If you have a short hose on the right, the "appropriate" hose routing would be straight up into an angled adapter of some kind (fixed or 360*). This, however, would make it very difficult (if not impossible) to share that hose at all. Also, since it wouldn't be behind you at any point....dropping it would mean it falling to the end of its hose. Routing the short hose from the left side allows the hose to come across your body. Dropping it would mean it falling infront of you. Also, it could be used temporarily for ooa situations. Using the breakaway-bungee method, that hose can be donated (if ripped out or if longhose is caught) and the hose that was behind you can be used to give some distance between the two divers. This can be fixed by deploying the longhose and giving it to the ooa diver. That's how i see it....so there are valid arguments for short hose left, long hose right. I haven't seen any the other way except for a shorter hose and less hose behind you.
The principle isn't just a working regulator. You also want to donate a hose that gives "breathing room" (pardon the pun) between the two divers. If you donated a 6" hose, it wouldn't be very comfortable even in mild ow conditions. The long hose, both for backmount and sidemount, came from overhead training. The 7' longhose maintains its benefit in a cave regardless of where you place the tanks. A longhose (5' or 7') is beneficial in ow diving as it allows both divers to maintain more comfortable positioning while proceeding with the dive. As to the "working regulator" principle, i think that one issue with using the bungeed octo as only the backup is that it never gets used. In any silty/muddy/sandy/salty situation, repetetive dives in sub-premium conditions can cause a bad buildup. Yeah, you're donating a good reg....but you have no guarantee your octo breathes at all, much less well. With independent doubles, you're may not donate a hose that you were literally just breathing off of.....but you had been on that dive, no more than a few minutes prior. That, to me, is much more comforting than one reg potentially being completely inoperable. Ooa divers, even well trained tech/cave divers can freak out when they hit that last breath. You donate your long hose, switch to your bungee and find it's not giving you gas but your buddy won't give up your longhose....what do you do? With independent tanks, you breathe off of both consistently. Also, independent tanks makes you much more likely to get an equivalent backup. How often do you see divers buying cheap 2nd stages because they're "just octos and they'll never get used." i've seen more than one tech diver do that.....and i've seen a ton of longhose divers do that.