The reason people like doubles is due to the redundancy. On a proper set of doubles, there's a knob between the two tank valves. That way, if something goes wrong with one of your tanks, you can isolate them. This means that you can dive it as if it were one big tank normally, but split them up if something goes wrong. A proper set of doubles also has a set of stainless steel bands between them, allowing you to easily mount them to any BPW.
The tanks you show are practically one big tank, with two differences: Many say that the two small tanks fit closer to your back and are wider and are therefore more stable. But they're much more expensive than a single big tank.
If I were you, I'd either get a big, cheap tank or I'd get proper doubles.