Snowbear:My back has been seriously messed up for about 4 1/2 years now.
To lift the doubles, I get into the harness, roll up onto my hands and knees, then stand up. I *can* squat down and lift them by the valve on one tank and the bottom of the oposite tank, but the getting into the harness method has proved more reliable for not causing the constant pain I already have to get even worse.
That's how I usually do it. Fill stations with benches are nice and in Florida (Bill R's) we just pull in the drive and fill them while they sit in the truck.
If I have to lift them without the harness I use 2 people if possible and if not I grab a tank neck in one hand and the bottom the the sane tank in the other (so the tanks are horizontal and lift with my legs holding the tanks close to my body. That lets me get them high enough to get them onto most tables or benches.
Wearing them is the easiest way to move them but watch it on hills, stairs, slippery surfaces or climbing in and out of boats because falling with the doubles on could be bad news.