If it's a short walk to the water and no benches are available, I will just set up the tanks in the back of my SUV (a Jeep Liberty) and then set back down in the back of SUV before taking off the tanks. It minimizes the lifting and tank futzing, you never have to lift from ground level, and you can just lean forward and then use your leg muscles to stand upright with the vehicle available to stabilize your self.
If it's a long walk, or involves walking down a rip rapped shore line, you could consider independent doubles to allow you to carry the tanks and assembled regs separately and then assemble them in travel bands at the water line.
Of course, now that sidemount is so widely available, a sidemount system makes much more sense and gives you a variety of options. Carrying both tanks in sidemount to the water is an option and is easier/safer than back mounted doubles as your center of gravity is lower. If you plan to do that, the main requirement is to ensure you have an upper bolt snap so that the weight of the tank is not carried on the bungee (as they can break). You also have the option of using the harness to carry one tank at time to the water, or to just carry the tanks like you'd normally carry a single tank. With that approach, it's a lot like riding your bike in low gear - the effort is low, but you have to expend that effort for twice as long.
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Using smaller doubles for back mount is problematic and not always efficient. For example, I can double up a pair of steel 72s and have a fairly light set of doubles (29 pounds each, 58 pounds total and about 62 pounds total weight with an isolator manifold and bands), however at 2250 psi they only hold 65 cu ft each for a total of 130 cu ft, and an X8-130 only weighs 43 pounds. With AL 80s the doubles weight is about 67 pounds for a total of 154 cu ft - still not much more gas than a single 130 that weighs 24 pounds less. And that's not counting the additional weight of the extra first stage with the doubles.
You end up having to go with double X7-100s or larger to really start to see the pay off with doubles as they only weigh a couple more pounds than double AL 80s but have an honest 200 cu ft of gas.
In my opinion, X7-100s (33 pounds each) are the sweet spot for shore diving doubles - only about 5 pounds heavier than double 80s, but 46 cu ft more gas. Faber LP 95s (38 pounds each) pumped to 3600 psi (and 258 cu ft of gas) run distant second as they are 10 pounds heavier, but offer another 58 cu ft of gas - and are 10 pounds lighter than double 130s (260 cu ft of gas).
