SmpleGreen, that's really the point here. In addition to the helpful advice and observations others have made, the fact is, at this point in your diving, you should be well past the point where you walk into a dive shop to rent gear and have them eyeball your frame and hand you a medium (or x-large) wetsuit and 15 (or 30) pounds of lead. You need to understand what you're diving, and you can't just take the attitude that it's OK to rely on someone else to "tell you what you need," even if the gear is all new to you. What is the buoyancy of the exposure protection used? Of the steel tanks and the stage? Of the gas inside of the tanks at the start of the dive? Of the backplate and associated rigging? Of the suit once it's compressed at depth (at 3ATA? 5ATA?)? If the shop you rented from didn't go over this, your instructor most definitely should have, before you ever get in the water with it, and particularly since you've never dived the equipment before.
But in the first instance, and more importantly, the most troubling aspect for me is that (at least as presented in your posts), you yourself never asked these questions of anybody. Of course you won't know the answers, since the equipment is new to you, but these are the same questions you needed to ask in order to properly weight yourself and balance your rig when diving a single tank, and they're ever so much more important now.
I hope you don't feel like you're getting dogpiled here; we're trying to help, but at least some of us feel that some of what we're hearing in this story is very disturbing, and it really seems like you're taking on way too much new stuff (gear, instruction, skills, conditions) at once, and really need to slow down quite a bit. Looking over my logbook, I had about 50 dives in doubles before approaching 90ft, and before putting them on for the first time, I spent a good amount of time researching the equipment and buoyancy characteristics so I could have a ballpark estimate of proper weighting and a basic understanding of the gear I was approaching. While you certainly don't need to take the same approach I did, my impression from what you've presented is that these dives were way too much on the other side of caution and preparedness for comfort.
Gombessa... in terms of taking on way too much new stuff, I think you are absolutely correct. It is clear I got in over my head and need to back off and take this more slowly.
As far as relying on someone to "tell me what I need", I think all education at first is relying on someone else to instruct you in how to perform any task. When I was an open water student, the instructor could have told me that I needed to dive with 6 steel tanks, and I would have believed him. We are all relying on someone else to tell us what we need at first. Someone told you what you need to dive with when you started out as well.
Technical diving is in many ways like starting out as an OW student again. I have no familiarity with backplate and wings, how to set them up, how to dive them, etc. That's why I took the class in the first place. Once I learn those things, then it will be up to me to decide how I want to dive my rig. Until then, I am by definition relying on other people to instruct me in how to figure out what I need.