Note always. Imagine having two bands next to one another. They both have to pass through the (plastic) backplate twice to form a figure 8. That means at least three slots in the backplate. Mostly they only have two slots, maybe two sets but one set above the other not side by side.I saw the pictures and that is exactly what he had. So I assume you just attach the twin bands to a single-tank backplate and presto.
This is hard to get over without a picture. You can sort of see in the third picture, from the rear, here AP COMMANDO | AP Diving