Ok, I'm gonna be the stick in the mud here, but its well-known around here what I think of most of the classes and agencies.
Doubles. First question is why do you want to dive them? If you're diving in the range of > 60 < 130 or so, and are carrying a pony, and would like something a bit more secure, then you have a reason to dive doubles.
If you're doing it as a means to get familiar with them before undertaking serious technical diving, then there's a good reason to dive doubles.
If you're doing it because you are ALREADY doing deco dives on a single, and you've had a few too many thoughts go through your head of bending the bejeezus out of yourself due to a problem with a reg or similar event, you have a good reason.
Otherwise, think long and hard about it. Doubles are heavy, for one. My sets of Double 72s weigh 65 lbs empty, and 75 lbs full of Nitrox. My Double HP100s weigh 90 lbs full. They are a pain in the butt to move around on dry land and ALWAYS WILL BE. In fact, I have a hand truck at my house that I use to move them around my fillstation area!
OK, you've decided you're going to do it anyway.
First things first. A set of Doubles can be very negative. Choose carefully based on your exposure suit and plans. Remember, the deal is that you have to be able to swim up the kit from the bottom, with a full load of gas, and a dead wing.
The usual limit for most people on this is somewhere around -20lbs or so. Beyond that you WILL SINK UNCONTROLLABLY, and if in a place without a bottom, that would be bad!
The usual "DIR parrot line" is "never dive steel doubles in a wetsuit." I will tell you right now that this is absolute, complete, unadultered crap. I do this all the time without violating the above rule and there is ABSOLUTELY NOTHING WRONG WITH IT. In fact, there is plenty wrong with double AL80s, not the least of which is that they are VERY butt-light and can make trim a real nightmare.
Do the math. An HP100 is about -1 empty. Double HP100s are -2 for the tanks, and another -3 for the bands and manifolds, or a total of about -5. Double AL80s are about +5 empty, all in the butt, which can be extremely uncomfortable.
Assuming you're diving a 3mil wetsuit, and are an average size man, the suit + booties are probably +7 or thereabouts. An AL plate is -2 Double HP100s are -5. Guess what - that's a balanced rig. It will be -20 or so at the bottom full, as the wetsuit will lose most of its buoyancy and double HP100s hold 15lbs of Nitrox (less if they're full of Trimix.) If you can't swim up -20, you can't carry 15lbs of gas and you need smaller tanks. If you can, then the suit you wear is not relavent.
Having done this computation (and you NEED TO COMPUTE IT for your combination - get a fish scale if you need and weigh the freaking tanks in the water!) the other "big deal" is valve skills.
The reason you dive manifolded doubles is to have redundancy. You only have that if you can shut down a leak before a significant amount of your gas bleeds out into the water. If you cannot do this, then the doubles are useless and a big single is a better choice.
OK, what does "reasonably quickly" mean? It means that within 15-30 seconds you must be able to stop ALL loss of gas, and ideally, within 5-10 seconds you should be able to isolate. Why? Because the worst-case scenario is a blown LP hose that will dump your tank at your first stage's "open flow" rate - which is stupid high. If you cannot protect at least half your gas in that scenario, you die anyway, doubles or no doubles!
I've given this a LOT of thought. In the event of a MAJOR leak the first priority is to close the isolator unless you are CERTAIN that (1) which side the leak is on, and (2) that its NOT coming from the tank O-ring or manifold itself. If in doubt, close the isolator first!
I KNOW there will be people who will argue this, but consider the facts. Most leaks are small. In that case, which you close first doesn't matter, as the gas loss rate is not all that high. So whether you turn one valve or two doesn't make a bit of difference to the outcome. If I lose an extra couple of cubic feet, its no big deal.
But consider the worst case scenario - a catastrophic failure of the crossbar O-rings, or a total tank O-ring failure (extrusion, etc). Or, for that matter, a blown LP section on your reg (e.g. hose fails at the swege, turret blows off, etc.)
In the case of a tank or crossbar O-ring, only shutting down the isolator protects any of your gas. Closing a post does nothing, except waste time, and time you ain't got.
If you lose a turret or LP hose at the swege, you can dump a tank - a full tank - inside of one minute. Don't believe me? Try it sometime; remove a hose, jack the reg to a tank, put on some hearing protection, make sure the tank is secure so it doesn't fall over from jet effect and turn it on with your stopwatch. Be ready to be amazed.
Now given the chaos you're going to be experiencing right about then with the gas flow involved and the huge bubble clouds and noise everything; if you get it wrong as to which post is blown you may be screwed. Your immediate priority is to insure that you have something to breathe. The only way to KNOW that this will be the case is to close the isolator. Having done that, you then close what you THINK is the bad post. If the leak stops, you're right. If it stops and so does your breathing gas, you switch to the other side. If it DOESN'T stop you guessed wrong; turn back on the good one and shut the other one off! In any event you know that you protected at least half of your remaining gas immediately, so you're not screwed even if by the time you get it shut down the dead post has bled off everything underneath it.
Now the usual drill for "practice purposes" is to (1) close the right (main) post and breathe down the reg, (2) switch to backup, (3) open the right post and purge the reg to make sure its really on, (4) close the left post and breathe it down, (5) switch back to main, (6) open the left post and purge the reg to make sure its really on, and (7) close and open the isolator. The usual claimed "metric" for being able to do all this is 2 minutes; in a real "oh no" situation the real time to actually close that isolator is more like 10 seconds, and then another 10 to close the offending post. Unless the leak is small, I argue that you close the isolator first in a real emergency.
You need to make VERY sure that you can reach the valves and do all this, in "fighting trim" (with all your gear on, in the water) almost instantly if necessary. Practice, practice, practice, and do it in a pool that's shallow enough that if there's a problem you can just stand up! If you can't reach and manipulate your valves with reasonable efficiency, you have ZERO redundancy and in fact have ADDED risk, as there are more places you can get a leak with a doubles rig than a singles one!
Don't be surprised if it takes you a while to get the trim and feel down and be comfortable. It did take me a while.