I was wondering are there any flaws with running a 7' primary hose and then running your backup in the manner described above? Curious as to others thoughts because pictured in my head it seems pretty good.
You can do it. As several have indicated, I don't know WHY you would want to do it. If you happen to have a 40" hose that you want to use, and don't want to buy the (usual) shorter hose used with the bungeed necklace reg, it might make sense. But, the cost of adding the elbow more than offsets any savings of using a hose you already have. If you are thinking that it would give you an option to donate either the long hose or the shorter hose alternate, I don't see the point. It is already bungeed and you would have to lift it over your head to donate it.
Maybe, we are missing an important part of your reasoning, and would benefit from a better understanding of why you think it seems to be a pretty good idea. I am not criticizing the idea, I would just like to better understand your thinking. The title of your post comments on 'running both hoses under you right arm', which confuses me. I don't think of my long hose as running under my right arm, although I guess that technically it may well be doing that. Rather, it runs down my side, then under an object on my waist strap (hard-backed pouch, reel, canister light) or under the waist strap itself, then up across my chest.
xdrjones5:
it would make it more comfortable in the event you had to donate the long hose and switch regs.
This is a part I don't understand. If you mean breathing off a second stage attached to a hose routed under your right arm is more comfortable than breathing off a second stage attached to a hose that is routed over your right shoulder and around the right side of your neck, I would disagree. There is essentially nothing uncomfortable about the latter (providing you have a proper length hose).
xdrjones5:
I would not run it on the left because it would interfere with my drysuit hose.
Although the drysuit hose is frequently routed under the left arm and across the chest to the DS inflator valve, and there is some potential for conflict if you ran your alternate hose under the same arm, I am not sure this isn't manageable if the hoses are set up correctly.