WIF,

Yes, the learning curve can be steep and maybe a bit painful, as the double paddle uses a little different set of muscles than a single paddle does.
I tried with a kayak paddle while still a tandem paddler. Even from bow/stern I had to windmill the double to avoid hitting the gunnel's every stroke. Exhausting to be sure.

When I went solo, I did some research and found that indeed you need a much longer paddle .
I use a Mohawk 9' paddle that they sold as a canoe double paddle. Not the lightest but for $50, it's tough as hell and until the weight becomes an issue, I'm happy with it.

Another factor in using a double over a single or both as you/I do, IMHO has a lot to do with muscle memory.
People that are able to paddle regularly like any repetitive motion athlete, develop muscle memory.
Putting a double in  their hands is like giving a golfer/tennis player a completely different set of clubs/racket.

In my case, I don't get to go often enough to be locked in to one particular set of muscle memories.
Whether double or single, the first few days on the water I'm struggling to get a rhythm anyway.

Not for everyone, but IF you can make it work, it gives you a good tool that excels in certain conditions.