I started sidemount diving with rented tanks myself. Only got my own after about a year.The part of you logic that still fails to explain itself is the idea that a continuous bungee loop (which means it's running from side to side around your back (and thru the shoulder plate (or an additional small back plate). Sure, when you load your left post, you are going to have plenty of slack in the bungee, and when you load the right, in theory, everything will tighten up. But, BUT!!! if that bungee breaks, and/or you are trying to match this method to diving in almost any Asian country (including the Philippines) you'll find that most shops don't have long stems or left right tanks. That means that if you try looping on the short stem (possible if you are really good with your hardware setup), if one tank pops and drops, you lose the other too, in almost EVERY case. In the case of a broken bungee, you are basically screwed.
In my opinion you can use any valve with the single bungee.
Sometimes you have to decide to either have secure attachment or optimal comfort, but it always works somehow.
I have never had problems when a bungee slipped.
The hoses catch the tank and I reattach the bungee one handed.
When it breaks I replace it immediately (only happened to my ones without provoking it on purpose). Grab it from the pouch, attach, continue.
The second tank never dropped, even if I tried to provoke it, it seems quite impossible to me to happen by accident and it is often rather hard to get the second tank untangled when the bungee breaks and losses tension.