A design very similar to how a door hinge is constructed, three bits of tube held in line with screw bar or a turned pin, the top and bottom sections are welded to one gate and the middle to the other.
I'm interested in these hinges as well for gates on the side of my drive so they will open either way but fold back on themselves yet can't think how to engineer.
This the best picture I can find of how they work -
Actually I've got some cheap wood door hinges that will never be used somewhere so might tack them together to "prove concept"
For gates I reckon angle iron could be used so gap between then would only be a little more than thickness of the metal.
They would need to be pretty robust because the weight of the gate will make the top hinge 'fan out' like the pic and will eventually break from the stress.
Ok so I have bought 4 barrel hinges my problem now is how do you stop the barrel hinge getting lifted off. I have set the hinge up with some scrap to show you what i mean.
Mate. Those tabs are massive my barrels are only set 16 mm from the face
Your way overdoing it and the swing will be greater
Also you want the two leaves to fold onto each other
Just enough so you can just get the welds in
at a push