Another take on "one fly" and tenkara
After the Tenkara Summit last September, I had an extensive discussion with Dr. Ishigaki, the most prominent "one fly" advocate, about what is and what isn't tenkara. We covered a lot of "what about"s - (what about weighted nymphs? what about dry flies? what about bluegills in ponds? etc.) What it finally came down to was the only thing he was adamant about was "one fly" but not the one fly you're thinking about. He wasn't talking about using only one pattern all the time, he was talking about having only one fly on your line at any one time. Fishing a nymph is (or can be) tenkara, fishing a dry fly can be too, but fishing both together definitely is not (at least in his view).
The use of one fly or multiple flies might in fact be the biggest difference between tenkara and pesca a mosca Valsesiana, which uses three or four flies at a time (with a long rod and line tied to the rod tip).
We didn't get into why only one fly at a time, and there is evidence that at one time Japanese fishermen used multiple flies on their lines.
However, over the weekend I learned of a potential problem that could definitely cause someone to fish only one fly at a time. A guy emailed me and said he broke his rod tip. He had been fishing two flies and had gotten the lower fly snagged in a spot that was too deep and too swift to reach the fly. He did what we say he should do, he got as close as he could and then collapsed the rod so that he could reach and pull back on the line to break off the snagged fly. Unfortunately, while he was collapsing the rod a large brown grabbed his second fly. A tenkara rod can handle a big fish if it can spread the strain over the full length of the rod, but when it is mostly collapsed it takes very little sideways pressure to break the tip.
I've often felt I was courting disaster when collapsing the rod and allowing the line and fly hang in the current downstream. Sometimes fish do hit a fly held in the current like that. If a nice fish hits your fly when the rod is mostly collapsed it could easily break your rod. I hadn't ever thought of a similar problem when fishing two flies and getting one snagged, but obviously it can happen and it is not at all clear what can be done to prevent it, other than pulling straight back on the rod without even trying to collapse it and reach the line - but that risks getting a stuck tip from pulling tightly enough to break the tippet.
Maybe that's why tenkara is a "one fly" method.