You may be opening a can of worms with this topic, a lot of passionate people in this forum.
some points:
- a canoe double blade is not a kayak double blade
- the blade shape is different
- the blade area (size) is about half of a single blade
- the length is substantially longer (260 - 280 vs 200-220)
- canoe width plays a big role in suitability, the wider the canoe, the longer the double needs to be
- canoe width is the width where you will be using the blade, the wife and I have used doubles in a tandem Freedom 17, the ends are narrow and it worked wonderfully ... again, she used both and preferred the double ... fit the equipment to the user
- the stroke when using a canoe double blade is completely different than a kayak stroke (or should be)
- the blade is designed for shallow entry, not the steep angle of a kayak blade, if you get water in the boat, it's user error
- it is not "twice as efficient", it is more efficient, simply because every stroke is a power and correction stroke
- it does result in faster stroke cadence, as every paddle movement is a stroke ... a single blade spends 50% of the time recovering
- in heavier water, correction happens as a matter of stroking, making head way and doing corrections as a matter of paddling
- you still have to do the work, the paddle isn't magic
- each stroke is easier, as the blade is smaller ... but you are doing more actual strokes
- since the paddle arc is out further, the blade also moves through more water ... delivering more "push" per stroke
- you definitely have to fit the paddle to the user
- select the double blade with the same care you would for a single blade
- too short, too long are both going to be nightmares to use
Just to be clear, I have both types (lots of both types). make both types and use both types. When I trip, I take a double and a single ... when I hit the lake it is usually with the double ... I find it best for putting in miles or handling wind and waves. When I have smaller water or am fishing, it is usually the single I reach for, it gives me more flexibility and uses less real estate to maneuver (IMO).
Your paddle is the same as any other piece of equipment, there are different types to be used in different situations. Even if the environment doesn't dictate a certain style of equipment, you will likely gravitate towards a "type" because it suits you better. To really answer your question, you need to get the blades in your hands and see what you think and how they work with your canoe.
Brian