CLick on the picture for more shots of me experimenting .
While paddling around in Haleiwa Harbor two summers ago, My son David and I decided to splice two paddles together and make a double kayak sort of paddle.It was taped together and simple, the result was obvious. This paddle was propelling the board through the water quicker, at a more consistent rate.
I made several double bladed paddles to experiment with. I paddled in the wind, not so good. I paddled in chop, not so good. I surfed, ok but unwieldy. I paddled in smooth conditions for exercise, a lot of fun, and fast.
 

Details
I paddled for distance and found wrist fatigue the big problem. To manipulate the paddle takes a lot of strength and odd hand positioning.
Race Hawaii is selling the DRG line from Fred Lane. Fred developed the Dynamic Rotating Grip to deliver higher performance and help with wrist fatigue in Kayaking. The DRGs work just fine for standup too. I have found these grips to be cost effective at $40.00. They can be applied easily to a doulble paddle.They make a double bladed paddle a lot of fun.