I dove independent doubles for years for boat/travel related reasons.
Keep it simple in terms of gas management. Breathe 1/3rd off the left tank, then switch to the right tank and breathe 2/3rds off the tank, then switch back to the left tank for a final 1/3rd.
That leaves 1/3rd in each tank as a reserve and equally importantly, it leaves 2/3rds in each tank at the point where you turn the dive - where you have so far used two 1/3rds of the four 1/3rds you'll use during the dive. It also keeps the gas switches to a minimum and leaves you on the long hose on the right tank for the portions of the dive where you are farthest from the exit or deco stop and where gas sharing on the long hose is most critical.
As for configuration I am a big fan of a long hose primary and bungeed octo. It will serve you well with any back mount doubles you may eventually use and I have yet to find a tech instructor too unhappy with it.
In this case, you would have on the left regulator:
1. An SPG on a slightly shorter than normal lenght hose. (it needs to reach to the left hip d-ring without much extra lenght.)
2. The bungee backup reg on a 22-24" hose running behind your neck and then around the right side of your neck to hang just under your chin.
3. A dry suit inflator hose if using a dry suit (30-31" usually works fine with under the left arm hose routing to a center of the chest mounted valve.)
On your right regulator you will have:
1. the long hose primary (7' hose) routed down your right side toward your waist (tuck it under acan light or tuck it in the waist band) then diagonally across your chest, over your left shoulder around the back of your neck and then to your mouth. This reg should have a bolt snap attached to the hose near the second stage to let you clip it off to the right shoulder D-ring when you are not using it. (Take a tank neck o-ring and sqeeze it to an elongated oval and pass it through the eye on the boltsnap, then pass the LP hose through the rsulting loop in the o0ring on each side of the bolt snap.)
2. The wing inflator hose - lenght will vary a bit but it will probably be in the 18-21" range with most normal lenght corrogated inflator hoses. This will route across the left and then down along the corrogated hose and left shoulder strap. (Most divers use a couple loops of bicycle inner tube to hold the corrogated hose next to the shoulder strap so it stays put and does not dangle.)
3. A second SPG on a shorter hose (18-20"). My preference was to route this through a neoprene shoulder pad on the right shoulder strap. It left the right tank SPG peeking out from under the lower end of the pad right above the right shoulder D-ring where it was easy to see, streamlined and out of the way without having to clip it and have a right hip d-ring.
All the hoses should to the extent possible route downward to prevent line traps or hoses that stick up or out away from you.
You want a BP/wing - not a BC. Depending on the tanks you'll need a wing in the 40-50 pound range.
Aqua Explorers used to have their Ultimate Velcro Doubles bands and they were about the best travel bands around - as long as you replaced the plasctic cam bands with metal ones. But I think they have been discontinued. A good set of travel bands like those will keep the tanks secure and allow you to change the tanks out one at a time with no tools.
Diving doubles is not rocket science and provided you stay within recreational limits you can go slow in terms of picking up the management skills. A mentor who is familiar with doubles is very sueful, even if you take an intro to tec class - and I do recommend you find a tec instructor, but be aware many are very picky about configuration and may not be in favor of independent doubles.
Tank wise, double 80s are hard to beat for a new doubles diver as they are pretty stable, not overly negative, and not overly heavy. The same is true for HP 100s or steel 72's as well.
Currently I do not take a subway to the dock, but I do live in an eight floor apartment. I tend to wear my doubles and then haul every thing else in a large plastic box on a small two wheel telescoping/folding cart I picked up at Home Depot for $25. I can also put the doubles on the cart and carry every thing else over the shoulder in a dive bag or in a dive bag with shoulder straps. Option three is to take the tanks on the cart and the other stuff in another roller bag.
All three may give you the means to use regular manifolded doubles if you eventually own tanks rather than rent them and allow you to take everything in one trip without killing yourself.
The major cautions are:
1. Doubles give you a lot of gas that can get you in a lot of deco trouble. Start slow, and stay with rec profiles (you should be able to get 2 dives per set) until you get proper training in deco procedures.
2. Instead of gas weighing 5 lbs the total supply weighs 10 pounds, so you will get twice the buoyancy change during the dive with double 80s as you are used to with single 80s. That means you are a lot more negative at the start of the dive - at least 10 pounds plus exposure suit protection. That makes buoyancy control more demanding and requires a lot more precision in terms of maintaining neutral buoyancy.
3. An AL 80 is about 4.4 pounds positive when empty. But after adding a reg and travel bands the net weight change will be about zero compared to a single 80. But while still in a quarry, confirm that you are still neutral at 15' with no air in the wing a with only 500 psi or so in the tanks. You want to ensure you are not over weighted with doubles and ideally you need to be able to swim it up from max depth in the event of a wing failure - or have an alternate source of buoyancy such as a dry suit.
4. If you use a dry suit, you need to use the wing to compensate for the extra weight in gas, not the suit to avoid having excessive air in the suit that can be hard to control.
5. A doubles configuration is a holistic thing. It needs to take into consideration the tanks, plate, wing, exposure suit and any deco stage bottles to ensure the rig is balanced, trims well and is well matched to the demands of the dives you do. This is one area where a mentor and proper training really help.
