I tie the second fly to the hook bend. I tie the clinch knot over my finger and then slip it over the hook bend. I originally used a dry fly with a nymph or wet fly dropper, but now use all kinds of variations; wooly buggers, soft hackles, emergers, midges and nymphs.
As far as distance between the two flys it depends on the depth that I want to fish each fly, water flow, and how I want to present each fly. It is no different than fishing a single fly, only you have to think how each fly will perform in the water.
A couple of weeks ago on the Caney I caught my first "double"; two fish at once on a two fly rig. It was exciting and not as difficult to bring to the net as I had thought it would be. Instead of two fish fighting against me, they seemed to fight against each other. Quite a thrill!
A couple of times this year I have also tried fishing three flies on some leaders that I bought. These surprised me in how easy they casted and I did not have any problems with tangles. I even fished them in the dark before daylight one morning. Obviously, I rigged them the day before. These are great for searching to see what the fish are interested in.