I did my first dives using doubled HP100 tanks on Friday. I bought my setup through the LDS, and the owner did a great job in guessing where the bands should be placed for my height. On the first dive, my buddy and I descended down to 15' to a nice rocky ledge where I could practice. The LDS owner was concerned about me rollling, turning turtle, and not being able to correct. This definitely was not a problem. I started by rolling to my left and right, and I could feel the extra inertia of the tanks. I then did a full roll, and found that you have to be deliberate about turning all the way around. After a few tries, I was able to barrell roll without problems.
I did find that even though I had done ~80 dives on my backplate, the straps were too loose by about 1". On the surface interval I tightened them up, and dives 2 & 3 were very good, and I was still able to get out of the straps at the end of the dive. By the end of the 3rd dive I think I was at about 85% of my old buoyancy control. The head to foot trimming of the tanks was good- I'm very thankful for the experience down at the shop. I did notice on the second dive when I clipped a 40 cu ft pony on, that I then picked up a head down trim which was not a problem when I used to dive a single tank + pony.
All and all with doubles, my main concerns prior to diving them were being able to swim without turtling, and reaching my valves. I found both of these to be non issues. I was able to get through two full cycles of valve shutdowns, while maintaining buoyancy at 40'. The one thing I'd emphasize for people considering doubles is the amount of weight out of the water. With the backplate, two regs, two flashlights and full gas load, I'm guessing the weight is around 100lbs. With singles, I could sit on the ground, put on the tank/plate/wing, and then stand up. Now, I really have to have the tanks standing on the tailgate of the truck to be able to get them on. I have some boat dives scheduled in two weeks, so I'm hoping that I can manage them on the boat. So far, I'm thrilled to be diving doubles.