There are many applications for fishing two flies at once. There are also a few ways to go about tying them on. Like most things, there's no single or correct way to do it, but there are ways that work better for certain setups and scenarios.
I use loop knots for streamers and large dry flies like hoppers and stoneflies. I find they allow the fly more play and give it more action. The same concept applies when tying a peice of tippet onto the lead fly for a trailing fly. Tie the tag onto the eye of the front fly, in the same spot as the first knot. This will give the front fly more action while being stripped through the water.
For a hopper dropper, I prefer to tie the tag onto the bend of the hook. Some will argue this knot slips off of the barbless hook, I don't find that to be the case. Many prefer to tie off the eye for the trailing fly, I find this tangles with he fly more.
For nymph rigs, I prefer to tie eye to eye. As I often fish jighead nymphs, this ensures the jig sits properly.
