One of the benefits of fishing with two flies is an occasional "two for one special." Take for instance my recent day on the Trinity River. I was fishing two nymphs: a bead head Prince and a Copper John. On my third cast the indicator darted one direction then another and I felt a fairly nice fish tugging on me. Nope. It was two fish.
I pulled my line to the surface and found two 10 inch Rainbows, one attached to each fly. They looked like twins. One cast. Two fish. Cool.
This isn't the first time it's happened to me. Believe it or not, it's happened to me numerous times. One day on the San Juan River in New Mexico it happened twice in one day! That was the same day I had a small Rainbow on the end of my line and a 20+ inch meat eating bow came out of nowhere and swallowed my smaller catch. That also happened twice on that day. I landed one of those fish and was accused of bait fishing. Unfairly. But, I digress...
What this indicates is that if you begin excelling at nymph fishing, getting those flies down deep AND fishing with two flies (sometimes even three), you WILL catch more fish. Frankly, there's no reason to ever fish with one fly. I can't remember the last time I had one lonely fly on the end of my line.
Call me an opportunist. Feel free. You would be right! Any opportunity to increase my catch rates--within the bounds of the law and fly fishing ethics--I am all for it.
Maybe someday you can enjoy a "two for one special." One cast, two fish. Double the fun. Enjoy!