I use a 9ft kayak paddle for my primary paddle in "most" conditions, with bent shaft as a back-up.

I don't disagree with SD or Mad_Mat, I don't worry about Karma( maybe I should), I just find the double paddle suits MY way of getting around.
I don't get to paddle as much as those guy's, once a year for 2 weeks, "maybe" some local weekend stuff.
I would say my canoe paddling skills are a C+, what I lack in technique, I usually can make up with power and endurance.
 The double paddle does work a different set of muscles and IF your a single paddler that spends more than 1 trip a year in a canoe, over the years  you've developed "muscle memory" from the constant repetitive single paddle strokes.
For me, and probably others that don't get as much time as we would like, whether re-learning the single or a double, the first few days are "awkward"  anyway.
I'm always amazed how better I feel and how smoothly I move along, the last few days, compared to the first one's.
 I got into the double paddle after my second solo trip, I ran into a Middle-age Canadian guy coming from Conk Lk, into Quetico Lk, heading for Beaverhouse take-out.
 The wind was strong that day, and he was heading directly into it, with his double paddle, just hauling the mail.
  He saw me and pulled along side. He was wearing a big strap-on back brace! I was really impressed, when he told me that he had started
on Basswood and that he was going in for MAJOR surgery when he got home.
I asked about the double paddle, he said that being a occasional kayak-er, he saw the "potential" in the double paddle for solo canoeing.
I laughed, and said the "traditionalist's" would be rolling over in their graves!
He said that he had been in a canoe since he was a little kid. He had a dozen canoes/kayaks between him/wife and kids. He raced them and did white water trips.
 He told me that providing you can adapt to the extra "load", that in "certain" conditions, like directly into or with a strong wind, you could really make time.
He also had bent shaft and used it like I do, for trolling, easy cruising, maneuvering in tight areas, fast currents.
 His attitude was that whatever worked best for YOU was all that mattered ;)
I figured if it was good enough for a guy like that, it was worth a try?
I tried it next trip and it worked for me. I work-out with the double paddle in mind, I don't find it more tiring or hard to get accustomed too, I'd have a relearning curve after a year off, no matter what I paddled with.
I wouldn't buy an expensive double paddle. Mine is made by Mohawk canoes, you need a 9 footer for canoe. I'd also try to learn the single paddle basics BEFORE getting into the double.
I see NO reason ( Karma a side) you can't use both IF it works for you :-/