I don't know anyone who teaches dual long hose. That's super weird to me. Who was your SM instructor? Someone in AZ?
The issue is that now you've always got to have your second hose clipped off, which means if you do have to donate, while the other guy has gas, now you have to unclip to get a reg in your mouth. That puts you at risk, and since your buddy is OOG, you're now putting both of you at risk. If you don't get that reg unclipped and in your mouth, you don't get gas, which means now you're now OOG, while your buddy is OOG but attached to you. Super bad day if you drown and start dragging your buddy with you. You've turned a single incident into a multiple victim incident.
Safest configuration in my mind is a bungee'd second of whatever length is required, long hose on the other side. Personally I long hose the right tank, routes across my chest and around my neck. Secondary comes from left tank behind my neck to a bungee just like backmounted doubles. If donating, long hose gets donated same as back mount, and the rest of the hose gets pulled from the tank, like if you were pulling it from a can light once all is sorted.
Having an unsecured second stage is never an option. Some people with reversible second stages will come straight off the tank into their mouth or to a bungee, but all second stages should be secured either by being clipped off when not breathing them, or on a bungee around your neck. Some guys won't bungee a second stage like normal, they'll clip their second stage to a necklace with a bolt snap. I don't do it, but some people like it.
Obviously if you're stage diving, you're going to have more stuff to deal with, and in that case, since you're breathing just your stage and not switching, once you drop it or breathe it down, you stow it like any normal stowed reg. Since you're never going to be donating a stage, when it's packed up and stowed it will be out of the way anyway.
